The efforts of many individuals helped contribute to the success of this project.  Foremost, I wish to extend the utmost gratitude to my advisor and friend Lou Bjostad, for without his support and insights none of this work would have been possible.

I would like to thank my committee members Tom Holtzer, Mike Antolin and Lonnie Wood for their sound advice and helpful discussions.  A special thanks to Mark Carter, Lorraine Carter, Derek Nabel, Matthew Carroll, Tim Burton, Ian MacRae, Scott Isard, Gail Kampmeier, and Rick Jachowske for their assistance.  I also want to express my appreciation to Laura McGrath, Julie Simmons and Laura Alexander-Sandigo for their patience and detail-orientated nature.

Lastly, I would like to thank my close friends and family for their love and support throughout this time.  The support and patience of my wife, Sue, has been invaluable.
 

 The present study was initiated to define the role of lipid utilization and its relation to flight metabolism and energetics in the  Russian wheat aphid (RWA), Diuraphis noxia.  Gas-liquid chromatograph techniques were used to identify eight primary fatty acyl components in the RWA. Dodecanoate, tetradecanoate, hexadacanoate, (Z)-9-hexadecanoate, octadecanoate,  (Z)-9-octadecenoate, (Z,Z)-9,12-octadecadienoate, and (Z,Z,Z)-9,12,15-octadecatrienoate components were identified and quantified from known standards.  The total amount of fatty acyl components increased in apterous RWA as the age of the aphid increased from first instar to fourth instar.  Prior to alate emergence, tetradecanoate was found to be the primary fatty acyl component.  The amount of the tetradecanoate increased with consecutive instars up to alate emergence and then decreased as the aphid aged.

 Experiments with precisely-aged aphids indicated significant differences in fatty acyl components between unflown control aphids and  aphids that were flown to exhaustion.  When the exact age of the aphids was carefully controlled, the functional significance of the amount of tetradecanoate present was correlated with the aphids propensity to fly.  Alate RWA eight to ten hours post-eclosion had the greatest amount of tetradecanoate present and the greatest propensity to fly, as indicated by mean flight duration times.  This is important because there have been no previous empirical demonstrations that the uniquely high proportion of tetradecanoate found in the insect family Aphididae is a principle source for flight energetics.

 Lipid profiles of three separate RWA field populations were compared; 1) RWA samples collected from wheat tillers, 2) RWA collected from a ground-based suction trap, and 3) aerial RWA populations collected by helicopter mounted traps.  Lipid analysis of  RWA that were trapped with a ground-based suction trap consistently demonstrated that these aphids had the lowest lipid reserves.  Lipid reserves, primarily tetradecanoate, were demonstrated to decrease as a function of age and prior flight history.  Lipid analysis of RWA that were suction-trapped indicated that these aphids were either older aphids, aphids that  had exhausted their lipid reserves in a long flight, or both.

 Field-collected, laboratory-reared RWA that were confined to petri dishes and less than 12 hours old had greater amounts of the 12 and 14 carbon fatty acyl components than did randomly collected field RWA of an unknown life history.  Aphids that were confined to petri dishes and more than 12 hours old demonstrated reduced lipid levels.  Lipid analysis of randomly collected alate RWA field populations demonstrated that aphids with high lipid levels were younger aphids (less than 12 hours old).  Field-collected RWA with low lipid levels would be an indication that these aphids are older aphids that may have dispersed into the area from other localities.

 Lipid analysis of populations of RWA trapped by helicopter at various altitudes suggests that the RWA does not use its lipid reserves during vertical ascent.  The mean lipid content of helicopter trapped RWA was significantly higher than suction-trapped RWA and field-collected RWA populations of an unknown life history.   Results indicated that it is likely that the RWA is assisted during vertical ascent by upward moving air currents and then uses lipid reserves, primarily the tetradecanoate component, to actively fly downward.
 

 The Russian wheat aphid (RWA), Diuraphis noxia (Mordvilko) is indigenous to southern Russia, Iran, Afghanistan and countries bordering the Mediterranean (Walters et al. 1980).  The RWA was first reported in South Africa in 1978, and by 1982 it had spread to all wheat-producing regions throughout South Africa (Walters 1984, Hewitt et al. 1984).  RWA first appeared in the western hemisphere in Mexico in 1980 (Gilchrist et al. 1984).  The RWA was first identified in the United States in Bailey County, Texas, in March 1986 (Stoetzel 1987).  By the end of 1986 the RWA had been collected in Colorado, Kansas, Nebraska, New Mexico, Oklahoma, and Wyoming, providing evidence that the RWA is capable of long range migration and dispersal (Stoetzel 1987).

 Since its introduction into the western hemisphere, the RWA has become the major pest of wheat and barley in the United States.  For the western United States, economic losses due to control costs and yield loss totaled $54 million in 1987, $130 million in 1988, and $92 million in 1989, with estimated total losses exceeding $650 million by 1992 (Lee 1992).  Control costs and crop losses due to the RWA are estimated to total 5% of the gross value of the annual crop (Hughes and Maywald 1990).

 Because of the enormous economic losses attributed to the RWA, a great deal of research  has focused on the basic biology and ecology of the aphid (Dillwith 1993).  Optimization of any control measures requires the ability to predict the population dynamics of the pest.  A comprehensive understanding of how RWA disperses between agroecosystems would aid in developing effective control methods.  The RWA dispersal may be localized or over long distances, aided by poleward moving weather systems (Johnson 1969, Loxdale et al. 1993).

 Flight energetic experiments were carried out in the present study as part of an investigation of flight duration and probable dispersal sites in RWA.   Lipids play an important role in flight metabolism, and are often consumed at a quantifiable rate (Fulton and Romney 1940, Liquido and Irwin 1986).  The fatty acid composition of insects changes corresponding to various stages of development and dietary constituents (de Renobales et al. 1990, Bergman et al. 1991).  Factors that influence variation in RWA lipid metabolism may lead to insights on the propensity of the aphids to disperse and their capacity to fly long distances.

 The insect family Aphididae (Homoptera) is unique in that tetradecanoyl fatty acyl groups comprise a large proportion of the fatty acid present in lipid extracts of many species.  Over 50% of the total fatty acid content of RWA is tetradecanoate (Edwards 1991).  The presence of the high proportion of tetradecanoate in RWA presents interesting biochemical questions about its possible use as a primary energy source for flight.  For instance, how does the fatty acid profile of the RWA change as it ages? Does the RWA accumulate tetradecanoate before the initiation of flight? Does the amount of tetradecanoate decrease as a factor of flight?

 As part of an integrated ecological and physiological study to examine the RWA migration pattern, the present study was initiated to define the role of lipid utilization and its relation to flight metabolism and energetics.  Experiments are described in this thesis which were designed to confirm the ability of RWA to use fatty acids, primarily  tetradecanoate, as a source for flight energetics.

Insect Flight Energetics

 One of the most important characteristics of insects is their ability to fly.  The ability of insects to disperse, or migrate, may be the primary function for which their flight ability has evolved (Wigglesworth 1963, Johnson 1963, Rainey 1965, Brodsky 1994).  Other interesting and sometimes complex behaviors associated with insect flight include courtship, feeding and escape.  The fossil record indicates that flying insects evolved at least 300 million years ago and does not appear to have evolved a great deal since that time, indicating that it has proved to be a remarkably successful physiological adaptation from the outset (Dalton 1977, Wootton 1986).

 Flight is a form of locomotion that is dependent on a high rate of energy metabolism.  Weis-Fogh (1961) compared the power output of insect flight muscles as being "slightly greater than that of a small internal combustion engine".  In order to sustain the considerable energetic demands required during flight, immediate sources of chemical energy need to be readily available.

 The biochemical implications of the high energetic demands during insect flight have led to extensive specialization of flight muscle to maximize power output.  It is the hydrolysis of ATP that provides the immediate source of chemical energy that ultimately causes muscle contraction (Crabtree and Newsholme 1975, Beenakkers et al. 1985).  The amount of ATP present is relatively small (five micromole per gram fresh muscle) compared to the rate at which ATP is hydrolysed during contraction (10-2000 micromole per min. per gram of muscle)(Weis-Fogh 1961).  If only five micromoles of ATP are available per gram, insect flight muscle would be able to maintain contractions for less than one second (Weis-Fogh 1961).  Therefore, during sustained flight activity, ATP must be continuously regenerated from ADP, requiring the metabolism of substances such as glucose, glycogen, fatty acids, triacylglycerols and amino acids for flight energetic fuels.  Each fuel has advantages and disadvantages (as discussed below) and insects have adopted particular substrates or combinations of substrates for optimum fuel selection in order to suit particular physiological requirements.

 Carbohydrates are used by many insects for the duration of entire flight episodes, but are used by other insects only to initiate flight (Cockbain 1961, Weis-Fogh 1952).  Use of carbohydrates for flight may be advantageous in some contexts because small quantities are stored in the flight muscles themselves and are therefore readily available for flight metabolism (Bailey 1975, Friedman 1985, Beenakkers et al. 1985, Van der Horst et al.  1978).  The primary disadvantage of carbohydrates as a flight fuel source is their bulky nature; for every one gram of glycogen stored, two grams of water are needed to maintain its hydration (Wheeler 1989).  It seems improbable that large amounts of glycogen could be stored and utilized for prolonged flight (Wheeler 1989).

 Carbohydrates are important as a flight substrate in Hymenopteran and Dipteran species.  As described by Beenakkers et al. (1985), the potential flight-substrate value of carbohydrates was recognized in early studies of flight metabolism that demonstrated glucose utilization during flight by Apis mellifera (Jongbloed and Wiersma 1934).  Chadwick (1947) obtained similar results for Drosophila, in which reserves of glucose and glycogen were depleted after flight.  Both investigations used measurements of the respiratory quotient RQ  (defined as the rate of carbon dioxide production divided by the rate of oxygen consumption) to substantiate their conclusions.  RQ measurements have been of value by virtue of the inference they permit regarding the nature of the substances undergoing oxidation.  An RQ value near unity indicates the oxidation of carbohydrates while lower RQ measurements indicate that fat deposits are being metabolized (Crabtree and Newsholme 1975).

 Measurements of RQ values in Lepidoptera and Orthoptera indicate lipid metabolism during flight.  Several species of Lepidoptera were found to have RQ values of 0.72 (Sacktor 1965, Zebe 1954).  After moths were gorged with glucose, the RQ increased to 2.0, but after flight the RQ value again dropped to the low value of 0.72, demonstrating that glucose is not a direct flight substrate for the moth species tested.  Further evidence of fat deposits consumed during flight is provided by the RQ of 0.72 for the migratory locust, Schistocerca gregaria, during sustained flight (Krogh and Weis-Fogh 1951).  Weis-Fogh (1952) further substantiated his earlier RQ measurements (Krogh and Weis-Fogh 1951) by demonstrating that in the first half hour of flight the carbohydrate content of the locust decreased, followed by a more gradual depletion of lipid reserves throughout the remainder of the flight.

 Amino acids, specifically proline, have also been implicated as an energy source for flight.  In Locusta migratoria (Kirsten et al. 1963), Glossina morsitans (Bursell 1963), Phormia regina (Sacktor and Wormser-Shavit 1966), and Leptinotarsa decemlineata (Weeda et al. 1979), flight leads to a prolonged decrease in the concentration of proline in the hemolymph.  Triacylglycerol and alanine from the fat body provide for the major renewable source of proline within these insects (Weeda et al. 1979).  The physiological advantage of using proline rather than diacylglycerols is that proline is water-soluble and avoids the metabolical expense of lipoprotein carrier mechanisms (Wheeler 1989).

 Three physiological types of flight muscle have been categorized; the carbohydrate-utilizers, as in dipterous and hymenopterous insects (Chadwick 1947); the lipid- utilizers, as in lepidopterous insects (Zebe 1954); and those that use a combination, as in orthopterous and homopterous insects (Krogh and Weis-Fogh 1951, Cockbain 1961, Liquido and Irwin 1986).  Lipid-utilizing species have a greater propensity to make long-range flights than non lipid-utilizing species.   For example, the carbohydrate-utilizing honeybee is rarely found more than 3 km from its hive and may be feeding as it forages (Beenakkers et al. 1985).   The ability of locusts to make uninterrupted long distance flights implies the presence of appreciable amounts of energy-producing substrates.   Beenakkers et al. (1985) provides the following example of the superior quality of lipid as a flight substrate:  A locust with a metabolic rate of 280 joule per gram per hour would have to oxidize 500 mg of glycogen for a flight duration of 10 hours.   Locust using lipid as a flight substrate would require only storing 70 mg for the same 10 hour flight.   This striking difference is due to the higher energetic value of lipid compared to carbohydrate.  Simple calculations show that one mg of lipid is equal to nearly eight mg of stored glycogen, and the compete oxidation of a long chain fatty acid (palmitate) yields 129 molecules of ATP, but glycogen only yields 39 molecules of ATP (Crabtree and Newsholme 1975).  Additionally, as mentioned earlier, glycogen is always stored with appreciable amounts of water.  Conversely, water is produced by lipid metabolism, perhaps providing an additional advantage to animals living in arid climates.   For reasons of weight economics alone, lipid appears to be by far the more desirable substrate for insect flight energetics.

 The mobilization of stored lipids in the fat body, primarily triacylglycerol, is controlled by the adipokinetic hormone contained in the glandular lobes of the corpora cardiaca (Mayer and Candy 1969).   Jutsum and Goldsworthy (1976) analyzed the flight fuels of Locusta migratoria migratorioides and concluded that the adipokinetic hormone involvement in flight metabolism plays a crucial role in controlling the supply of substrate to the flight muscles.   Their results supported Weis-Fogh's (1952) study showing that locusts use both carbohydrates and lipids for flight fuels.   The involvement of the adipokinetic hormone in flight metabolism may be an adaptation to the dual use of carbohydrates and lipids for locust migration.   During migratory flight, diacylglycerol is released in response to the adipokinetic hormone, and carbohydrate is spared for other processes such as trivial flight, feeding and walking (Jutsum and Goldsworthy 1976).

 Ward et al. (1982) found exceptionally large amounts of lipid reserves in the flight muscles of the triatomine bugs, Triatoma infestans, and Rhodnius prolixus.  It is probable that flight is responsible for the dispersal of triatomine bugs which is significant due to the bug's ability to act as vectors of Chagas' disease in South America (Minter 1977, Schofield 1979).   Total glyceride and triacylglycerol content was found to decrease significantly during sustained flight.  Oleate and palmitate are the major fatty acid components of these triatomine bugs and probably also the major fatty acids used for oxidation.   Additionally, Ward et al. (1982) found the carbohydrate content to be low in both bugs and concluded that the supply of energy for flight is provided almost exclusively by the oxidation of lipids.

 As in the triatomine bugs, the neutral lipids of the monarch butterfly are also composed mainly of palmitic and oleic acids (Cendella 1971).   Analysis of the monarch's potential flight substrates show that lipids are the most abundant component, comprising about 45% of the dry weight (Brown and Chippendale 1974).   The monarch was found to contain only trace amounts of glycogen (less than one mg/insect).   Brown and Chippendale (1974) concluded that neutral lipids are the principle reserve which the adult monarch uses for migration.

 The first comprehensive study of the lipid content of migratory homopterous insects was conducted by Strong (1963).  The 21 aphid and 6 leafhopper species analyzed showed a high proportion (nearly 30%) of triacylglycerol.  Triacylglycerol serves as a supply of diacylglycerol, monoacylglycerol and free fatty acids that can be used as flight fuels (Bergman et al. 1991, Dillwith et al. 1993).  The 21 aphid species analyzed also had a high proportion of tetradecanoate, a fatty acid that has since been recognized as being unique to this group of insects (Strong 1963, Bowie and Cameron 1965, Fast 1966, Stanley-Samuelson et al. 1988).

 The quantitative use of stored lipid reserves as flight fuels in aphids was first reported by Cockbain (1961).  Groups of Aphis fabae were tethered for defined periods of time up to six hours.  Batches of aphids were then dried and weighed, extracted with petroleum ether, and re-weighed.  Similarly, the amount of stored glycogen used during tethered flight by A. fabae individuals was determined by first extracting the glycogen and then quantifying it with anthrone reagent.  This procedure allowed comparison of the amount of glycogen and lipid reserves in flown and unflown aphids of the same age.  His results indicated that both fat and glycogen were consumed during tethered flight of A. fabae.  Initially, glycogen is  consumed as a flight fuel, but after the first hour fat becomes the primary flight substrate and is consumed at a mean rate of 0.005 mg/aphid/hour.  Cockbain (1961) concluded that 90% of the flight energy for A. fabae is provided by fat reserves.

 The flight fuels in the brown planthopper, Nilaparvata lugens, have also been shown to be a combination of carbohydrate and lipid (Padgha 1983).   However, unlike other migratory insects, the brown planthopper's pattern of fuel usage is apparently more flexible.   During flight, carbohydrate metabolism may continue for several hours.  Additionally, lipid utilization in initiated almost immediately, rather than commencing when carbohydrate reserves are near depletion (Padgha 1983).

 A rapid synthesis and storage of lipid was reported to occur prior to alate emergence in the corn leaf aphid, Rhopalosiphum maidis (Liquido and Irwin 1986).   The lipid content of alate R. maidis was found to decrease significantly with age which correlates with the capacity of alates to maintain tethered flight.   Younger alates containing higher lipid reserves tended to have longer flight times.   Liquido and Irwin (1986) demonstrated that R. maidis uses lipid reserves during prolonged flight and their data indicates that other energy substrates may be consumed when stored lipids are depleted.

Aphid Flight Behavior

 Aphids are polymorphic with alates generally being produced in response to environmental cues (Blackman 1981).  Deteriorating host plant quality, overcrowding, photoperiod, and temperature are also significant factors that trigger alate production (Dixon 1985). The environmentally-induced development of alates does not necessarily mean that these individuals will migrate.  Not all winged aphids fly, and not all fliers express the potential for migratory flight (Shaw 1970).

 Although aphids are often characterized as weak fliers, unable to fly into a head-wind of more than 2 km per hour (Kennedy and Thomas 1974), they are capable of long distance dispersal.  Perhaps the two most famous references to examples of long-distance migration concern the finding of aphids near the north pole by Captain Parry in 1827 (Parry 1828) and in the Arctic ocean by Elton in 1924 (Elton 1925).  Being weak fliers, aphids are commonly at the mercy of the wind, and may have specifically adapted exodus flight behaviors which enhances their ability to reach high elevations and wind-borne displacement (Kennedy and Booth 1963, Johnson 1974, Isard et al. 1990).

 Prior to departure, aphids are sensitive to light of short wavelength (less than 500 nm) and tend to seek the highest available point before taking off for flight (Kennedy et al. 1961).  After takeoff, aphids continue to demonstrate a strong positive phototaxis to ultraviolet-blue light.  This upward flight of Aphis fabae may last for only a few minutes or as long as 4 hours (Kennedy and Booth 1963).   Gradually, after a period of flight (0.5-4 hours), aphids become attracted to longer wavelengths (550-575 nm) and repelled by ultraviolet light (Kring 1969, Kennedy and Ludlow 1974, David and Hardie 1988).  Surfaces reflecting yellow and orange light (550-575 nm) act as a behavioral cue to initiate downward flight and to alight on vegetation (Kennedy et al. 1961, Kring 1969, Kring 1990).

 Long-range aphid migration is associated with particular weather patterns.   Long-distance dispersal from southern to northern latitudes is assisted by poleward surges of warm air (Johnson 1969).   During the day, vertical motion of air (convection currents) may exceed 1-2 meters per second and provide considerable assistance to the ascent of small insects (Farrow 1986).  The probable significance of upward convection currents in aiding the vertical movement of aphids is supported by the presence of inert particles and apterous insects trapped high above the ground (Glick 1939, Freeman 1952).   Observations by Isard et al. (1990) indicate that aphids aided by vertical convection currents move into atmospheric zones characterized by relatively high winds.  Strong winds in the upper atmosphere may provide a means for the long distance dispersal of weakly flying insects such as aphids (Isard et al. 1990).

 It is apparent that aphids are dynamic creatures with complex flight behavioral responses.  Assessment of aphid flight behavior is often difficult because of their small size (Loxdale et al. 1993).  Observational evaluations concerning the distance aphids actually travel is often guesswork (Smith and Mackay 1989).
 

Introduction

 The Russian wheat aphid (RWA), Diuraphis noxia (Mordvilko) is the major pest of wheat and barley crops in the United States.  Control costs and crop losses due to the RWA are estimated to total 5% of the gross value of the annual crop (Hughes and Maywald 1990).  A comprehensive understanding of how the RWA disperses between agroecosystems would aid in developing effective control measures.  Flight energetic studies may be a means to determine flight duration and probable dispersal sites.

 The immediate source of chemical energy necessary for insect flight metabolism is provided by the hydrolysis of ATP (Crabtree and Newsholme 1975, Beenakkers et al. 1985).  During insect flight, ATP must be continuously regenerated from ADP, which is commonly supplied from the metabolism of carbohydrates and/or lipids (Crabtree and Newsholme 1975).  The use of carbohydrates and/or lipids as an energy source varies among species, but in many cases the primary source for flight energetics is lipid (Sacktor 1965).  Lipid depletion following flight has been demonstrated in Circulifer tenellus (Fulton and Romney 1940), Danaus plexxipus (Beall 1948, Brown and Chippendale 1974), Schistocerca gregaria (Weis-Fogh 1952), Aphis fabae (Cockbain 1961), Dendroctonus pseudotsugae (Atkins 1966), Homorocorhyphus nitidulus vicinus (Karuhize 1972),  Pantala flavescens (Kallapur and George 1973),  Locusta migratoria (Jutsum and Goldsworthy 1976), Rhodnius prolixus and Triatoma infestans (Ward et al. 1982), Nilaparvata lugens (Padgha 1983), and Rhopalosiphum maidis (Liquido and Irwin 1986).

  The above observations indicate that stored lipids provide a convenient reservoir of metabolic energy for periods of sustained energy demand.  Several characteristics contribute to the advantage of lipids over carbohydrates as a source of flight energetics.  As compared to carbohydrates, lipids provide nearly eight times more caloric content/unit weight (Beenakkers et al. 1985).  For reasons of weight economics alone, this fact could be of profound importance for flying animals.  Lipids, upon oxidation, yield nearly twice as much water as carbohydrates.  In addition, lipids may be stored in anhydrous form, while carbohydrates are stored in the bulkier hydrated form (Downer and Matthews 1976).

 To determine the amount and type of lipid consumed during flight, the initial baseline level of lipid must be known.  The fatty acid composition of insects has been shown to change with respect to age (Stanley-Samuelson et al. 1988, Bergman et al. 1991, de Renobales et al. 1990).  Lipids play an important role in insect flight metabolism and are often consumed at a quantifiable rate (Fulton and Romney 1940, Cockbain 1961, Liquido and Irwin 1986).  Knowledge of the rate that lipids are used for flight, the baseline lipid content, and the quantity and type of lipid remaining in an aphid, may provide insights about the aphids' ability and propensity to fly.  It is the objective of this study to quantify accurately the different lipid classes in the RWA in relation to various life stages and tethered flight duration.
 

 Materials and Methods

Insects and Plants

 Russian wheat aphids were obtained from the Colorado State University (CSU) Insectary and from a field population near Briggsdale, Colorado.  Field populations were collected by placing wheat tillers showing signs of RWA damage into Ziploc bags (four-liter size, DowsBrands L.P., Indianapolis, Indiana).  These wheat samples were returned to the laboratory and examined for live populations of RWA.  Russian wheat aphids collected from the wheat tillers were immediately frozen for later lipid analysis.  Laboratory colonies were maintained on winter wheat (Triticum aestivum) TAM 107 variety.  The number of antennal segments in RWA increases with instars, 4 segments for first instar, 5 segments for second and third instar, 6 segments for fourth instar and adults (Aalbersberg et al. 1987, Olsen et al. 1993) and this key was used to age apterous nymphs to instar.  Batches of fourth-instar RWA were observed every two hours for emerging alates.  Newly-emerged alates were reared separately and aged to 8, 14 and 24 hours.  Apterous nymphs and adult alates of the desired ages were collected and frozen for later lipid analysis.
 

Tethered Flight Studies

 Because of the small size of RWA, considerable effort was required to develop an effective and consistent method to tether aphids.  A short, finely cut piece of transparent tape attached to the dorsum of the aphid's thorax and secured to an angled metal wire provided a sufficiently stiff support to hold the aphid when flying.  This design simplified attachment of the tape to the aphid and reduced the possibility of tangling the aphid's wings and legs in the tape during flight.  Tethered aphids were suspended and flown until they stopped beating their wings, and the duration of each flight was recorded.  The tethered aphids were observed continuously.  Aphids occasionally stopped flying but were immediately coaxed to resume flying by puffing air over them or by touching their tarsi with a fine paint brush.  Aphids were considered to have flown to exhaustion if they did not remain flying for at least one minute after repeated stimulation.  After termination of flight, aphids were immediately frozen for later lipid analysis.  Appropriate controls were analyzed to ensure that the attachment of the tape to the aphid did not contaminate the samples.

 Three tethered-flight experiments were conducted to investigate lipid utilization as a function of flight duration.  The first experiment was conducted with colony-reared alates of  unknown age.  These aphids were randomly removed from colony rearing cages at various times during the day and flown to exhaustion as described above.

 The second experiment involved aphids whose age was known to within 12 hours.  Colony-reared aphids were collected from cages early in the morning (approximately 7 A.M.).  Alates were cleared from the cages the previous night in order to help ensure that RWA collected in the morning would be newly-emerged alates.  Alates that were collected during the morning were randomly separated into three categories; a morning (A.M.) control set that was immediately frozen, a set that were used for tethering, and a third set that served as an afternoon (P.M.) control group.  The P.M. control RWA were isolated in empty dishes and frozen after all tethered aphids had stopped flying.

 The third experiment involved aphids whose ages were known to within one hour.  Flight duration and lipid utilization were determined in RWA freshly- collected from the field.  Wheat tillers from RWA-infested fields near Briggsdale, CO., were placed in Ziploc freezer bags (four-liter size, DowsBrands L.P., Indianapolis, Indiana) and stored in a refrigerator (5 C).  Apterous RWA were collected from the wheat tillers, placed in petri dishes with cut wheat leaf pieces, and observed continuously for alate emergence, allowing age determination accurate to one hour.  Batches of newly emerged alates were reared in petri dishes and aged to 8, 14, and 24 hours old.  RWA from each age group were flown to exhaustion and frozen for later lipid analysis.  Aphids of the same age as the tethered RWA were isolated in empty petri dishes to serve as unflown controls.  Unflown controls were frozen at the same time as their tethered counterparts of the same age.
 

Lipid Extraction and Analysis

 Methyl ester standards, tetradecyl acetate, and 14% boron trifluoride in methanol (14% BF3 in MeOH) were purchased from the Sigma Chemical Company, St.  Louis, MO.  All other materials were of reagent grade and were distilled to insure purity.  Pasteur pipets, 4 ml vials, and filtering materials were cleaned initially with dichloromethane.  The 4 ml vials used for methanolysis were further cleaned by heating the vial at 100 C for one hour with 1 ml of the methanolysis reagent (14% BF3 in MeOH).

 Methods similar to those of Bligh and Dyer (1959) and Morrison and Smith (1964) were used to extract individual RWA lipids.  An individual aphid was placed in a clean 4 ml vial with a Teflon-lined screw cap. One ml of 2:1 chloroform:methanol was added to the vial.  The aphid was macerated with a glass rod, and the extract was filtered and evaporated to apparent dryness with a nitrogen stream.

 Glycerolipids in aphids were methanolysed to create methyl esters derivatives that allows for chromatographic measurements of the fatty acyl component of the glycerolipid.  Dried lipid extracts were methylated with 0.5 ml 14% BF3 in MeOH (60 C, 1 hour).  The fatty acid methyl esters were isolated by adding 1 ml of hexane and 1 ml of water to the vial.  The vial was shaken and the lower water layer was discarded.  The hexane extract, containing the fatty acid methyl esters, was then washed twice with water (or until neutral as indicated by pH indicator paper), filtered, and concentrated with a nitrogen stream.

 The derivatives were analyzed with a Hewlett-Packard 5890 gas-liquid chromatograph (GLC) equipped with a Carbowax column  (30 m x 0.25 mm, 0.25 um film thickness) (Alltech Associates, Dearfield, IL.).  The column was temperature programmed from 60 C to 240 C at 10 C/min (1 min delay) for a total run time of 30 minutes.  Lipid classes were identified by comparing their retention times to those of known methyl ester standards.  Amounts of fatty acid methyl esters were determined from peak areas by using an internal standard (tetradecyl acetate).
 

Statistical analyses

 Fatty acyl components isolated from RWA were subjected to analysis of variance on  Microsoft Excel, Analysis Tools, version 4.0.  (Microsoft Corporation 16011 NE 36th Way, Box 97017, Redmond WA.  98073-9717).  Experiments in which ANOVA yielded significant results were subjected to Newman-Keuls multiple range test allowing for unequally sized data sets (Zar 1984).  Confidence intervals were used in order to make statistical comparisons between the percent flight times of field-collected RWA and RWA that were colony-reared (Pearson and Hartley 1954).
 
 

Results

Fatty acid composition of aged RWA

 In the initial experiment, apterous and alate RWA of known life stages were analyzed for lipid content.  GLC traces of known standards were used to identify eight different primary fatty acid methyl esters (Fig. 2.1).  Dodecanoate (12:ME), tetradecanoate (14:ME), hexadacanoate (16:ME), (Z)-9-hexadecenoate (Z9-16:ME), octadecanoate (18:ME), (Z)-9-octadecenoate (Z9-18:ME), (Z,Z)-9,12-octadecadienoate (Z9,Z12-18:ME), and (Z,Z,Z)-9,12,15-octadecatrienoate (Z9,Z12,Z15-18:ME) components were identified and quantified from known standards (Fig. 2.1).  Flame ionization detector (FID) responses to known amounts of each synthetic compound (200 nanograms) were determined, and were the same for all compounds analyzed.

 The total amount of fatty acyl components in apterous RWA increased as the age of the aphids increased (Fig. 2.2).  Prior to alate emergence, tetradecanoate was found to be the primary fatty acyl component in the RWA (Fig. 2.2).  The amounts of 16 and 18 carbon fatty acyl components remained relatively stable, but the amount of dodecanoate and tetradecanoate components decreased significantly in aphids as they aged (p=0.05) (Fig.2.3). No significant difference was found in the amounts of hexadacanoate, (Z)-9-hexadecenoate, octadecanoate, (Z)-9-octadecenoate, (Z,Z)-9,12-octadecadienoate, and (Z,Z,Z)-9,12,15-octadecatrienoate components in adult alate RWA aged between 0 and 28 hours.   The amount of the tetradecanoate component increased with consecutive instars up to alate emergence and then decreased as the alates aged up to 28 hours (Fig. 2.4).
 
 

Fatty acid composition of tethered RWA

 For randomly sampled, colony-reared aphids of completely unknown age, tethered flight studies demonstrated that the overall fatty acid composition of aphids that were flown to exhaustion did not differ significantly (p=0.05) from unflown control aphids (Fig. 2.5).  For these aphids of unknown age, the tetradecanoate and hexadecanoate components of aphids that flew for less than one hour was significantly less than aphids that were able to fly for periods longer than one hour (Fig. 2.6).  The predominant fatty acid present in tethered unaged colony-reared RWA that flew for over one hour was tetradecanoate (Fig. 2.6).

 For the second experiment, in which the age of the aphid was known only within 12 hours, no significant differences were found among colony-reared RWA collected early in the morning and separated into three categories, A.M. control, tethered, and a P.M. control groups (Fig. 2.7).  We noted that the variances were quite large for these groups of aphids whose age was known only within 12 hours.  Aphids that were flown to exhaustion and alates from the P.M. control group demonstrated similar lipid profiles.  The lipid content of the A.M. control group was generally greater, particularly tetradecanoate, but not significantly so (p=0.05).  The overall predominant fatty acid in this experiment was also found to be tetradecanoate.

 A total of 14 field-collected and aged RWA were flown between 15 and 165 minutes.  In striking contrast to the previous two experiments, this third experiment with precisely-aged aphids indicated significant differences in fatty acyl components between unflown and flown (tethered) aphids.  Significant differences (p=0.05) in each of the fatty acyl components occurred between unflown and flown aphids in the 8-10 hour age group (Fig. 2.8a).  Only the tetradecanoate component differed significantly between unflown and flown aphids in the 14-16 hour age group (Fig. 2.8b).  In the 24-26 hour age group, significant differences between flown and unflown aphids were found in the dodecanoate, tetradecanoate and hexadecanoate components (Fig. 2.8c).  Alate RWA in the 8-10 hour age group had the most tetradecanoate present (Fig. 2.9a) and the greatest propensity to fly, as indicated by mean flight duration times (Fig. 2.9b).  The percent flight times of field-collected RWA was not significantly different from the percent flight times of colony-reared aphids (Fig. 2.10).
 
 

Discussion

 One of the most noteworthy features of the results presented here is the significant difference in the amount of tetradecanoate in flown versus unflown aphids of known age.  When the age of the aphids was carefully controlled, the amount of tetradecanoate present can be correlated with the aphids' propensity to fly.  Alate RWA in the 0-8 hour age group had the highest lipid content and the ability to fly for the longest periods of time.  The fatty acid methyl ester levels of tethered aphids in the 8-10 age group were significantly less than unflown aphids of the same age.  The predominant fatty acyl component in this age group, tetradecanoate, was significantly less abundant in aphids that were flown to exhaustion.  These results indicate utilization of tetradecanoate as a major flight fuel for the RWA.  This is important because there have been no previous empirical demonstrations that the uniquely high proportion of tetradecanoate found in the insect family Aphididae is a principle energy source for flight energetics.

 The fatty acid composition of the RWA varies considerably among life stages.  This is concurrent with results reported by Strong (1963), Febvay et al. (1992) and Dillwith et al. (1993) and demonstrates that care must be taken to compare the fatty acid composition of aphids from the same life stage and from narrow age windows within a given life stage.  Lipid analysis of aphids of unknown ages provided valuable insights concerning lipid profiles and flight propensity.  However, direct correlations to the type and amount of fatty acid used as a function of flight were possible only after carefully aging individual aphids.

  As in similar conclusions put forth by Cockbain (1961) for A. fabae, Atkins (1966) for D. pseudotsugae and Liquido and Irwin (1986) for R. maidis, the results of this study indicate that the amount of lipid stored during the RWA apterous stage influences the aphid's capacity to fly.  Lipid analysis of randomly-collected RWA of unknown age indicated that aphids that flew for longer periods of time (over one hour) generally had larger lipid reserves.  We therefore hypothesize that migrating (transported) RWA are probably young alates (0-8 hours old) as is indicated by decreased tetradecanoate content and the reduced capacity of older alates to fly.

 The high proportion of tetradecanoate found in aphids is due to a specific acyl thioester hydrolase that terminates fatty acid synthase chain-lengthening at 14 carbons rather than at the more common 16 and 18 carbons (Ryan et al. 1982).  The results of the present study indicate that tetradecanoate is a primary flight fuel for the RWA, but, the overall physiological significance of tetradecanoate in aphids is unknown.  Bergman et al. (1991) found that the amount of tetradecanoate found in R.  maidis was higher when the aphids fed on poor hosts and lower when the quality of the host plant was good.  As the RWA winged adult aged, the amount of 14 carbon fatty acid decreased while the amount of 16 and 18 carbon fatty acids remained relatively the same.  Together, these results indicate that when conditions are favorable the RWA may divert energy into reproduction, and when conditions are less favorable, tetradecanoate reserves are accumulated and directed towards dispersal.  In addition, since aphids are relatively weak fliers, and for reasons of weight economics, the accumulation of shorter chain fatty acids (tetradecanoate) may be an acquired adaptation for increased flight capabilities.

 It is important to note that the present study attempts to provide fundamental information on the flight energetics of the RWA.  The data suggests that the RWA has a physiological threshold such that RWA are best prepared to fly approximately eight hours after eclosion.  Tetradecanoate is highest in these aphids (8-10 hour age group), and decreases thereafter.  Although older aphids are less likely to fly, flight behavior in the RWA may only function to displace the aphid from the wheat tiller it developed on.  If an aphid gets caught in a rapidly moving atmospheric system, and is transported long distances, then this may be related to chance rather than specific behavioral cues (Rankin and Burchsted 1992).  Aphids transported long distances would be younger aphids that originally had a greater propensity to fly.  Flight energetic studies can be a useful instrument in studying migration patterns of RWA, as has been demonstrated by Fulton and Romney (1940) in the beet leafhopper, Circulifer tenellus, by Chen (1983) in the brown planthopper, Nilaparvata lugens, and by Liquido and Irwin (1986) in the corn leaf aphid, Rhopalosiphum maidis.
 

Introduction

 The Russian wheat aphid (RWA) (Diuraphis noxia) is an important pest species for which information concerning dispersal dynamics is essential.  The distance over which individual RWA are dispersing is not known.  The RWA dispersal may be localized or over long distances, aided by poleward moving weather systems.  A comprehensive understanding of how the RWA disperses between agroecosystems would aid in developing effective control methods.

 Flight energetic studies may be a means to determine flight duration and probable dispersal sites.  The amount of lipid consumed as a function of flight may be used as an indicator of the distance a specimen has traveled (Fulton and Romney 1940, Cockbain 1961, Liquido and Irwin 1986).  The fatty acid composition of insects changes corresponding to various stages of development (Stanley-Samuelson et al. 1988, de Renobales et al. 1990, Bergman 1991).  Determining lipid utilization in the RWA may provide an indirect means to predict the RWA stage of development, prior flight duration and probable dispersal sites.

 The dispersal of insects can be separated into three areas, takeoff and ascent, horizontal movement, and flight termination and descent (Hendrie et al. 1985).  Behavioral responses to phototactic stimuli have been shown to be correlated with both takeoff, ascent, descent and flight termination (Kennedy et al. 1961, Kennedy and Booth 1963, Kring 1969, Kennedy and Ludlow 1974, David and Hardie 1988).  The movement of aphids is also influenced by environmental atmospheric conditions that may enhance aphid dispersal (Isard et al. 1990).  Depletion of lipid reserves could pre-condition aphids to respond to environmental stimuli and cues, increasing their propensity for flight termination (Hendrie et al. 1985).

  Earlier experiments in this thesis demonstrate the propensity for the RWA to fly depends on post-eclosion age which is also correlated with a significant increase in lipid content.  Experiments in this chapter were designed to compare the lipid levels of RWA from three separate populations: 1) ground-level populations collected from wheat tillers, 2) populations collected from ground-based suction traps, and 3) aerial populations trapped by helicopter mounted traps.  The horizontal movement of aphids can be monitored by aerial traps.  In order for aphids to be aerial trapped, aphids must first have flown, or have been transported by the wind, into and across these areas (Loxdale et al. 1993).  Previous experiments indicate that the RWA uses lipids, specifically  tetradecanoate,  for flight energetics.  In addition, the lipid profile of the RWA changes with age.  By comparing the lipid levels of RWA populations that vary in vertical distribution, inferences can be made about their previous flight history and stage of development.
 
 

Materials and Methods

Russian wheat aphid collection procedures

 Three major classes of sampling devices were employed in order to collect wild RWA specimens from the field; 1) samples collected from wheat tillers, 2) RWA collected from a ground-based suction trap, and 3) aerial populations collected by helicopter mounted traps.  Field populations were collected during June and July in 1992 and 1994.  Field populations of RWA were very low in 1993.  Because of the low numbers of RWA in 1993, no samples were collected for lipid analysis studies for this year.

 The first procedure involved collecting RWA from wheat tillers at a field site near Briggsdale, Colorado.  Wheat tillers showing signs of RWA damage were collected, placed in Ziploc bags (four-liter size, DowsBrands L.P., Indianapolis, Indiana), and returned to the laboratory.  Alate RWA collected from the wheat tillers were immediately frozen for later lipid analysis.  Apterous RWA were collected from the wheat tillers, placed into petri dishes and observed for alate emergence.  The petri dishes contained several blades of wheat and a moist piece of filter paper.  Alate RWA were removed from the petri dishes and immediately frozen for later lipid analysis studies.  The procedure for rearing alate RWA was identical for both 1992 and 1994 populations except that the 1992 alate samples were aged to 12-24 hours old while the 1994 alate samples were less than 12 hours old.

 The second sampling procedure employed an 8.3 meter suction trap (Allison and Pike 1988) to collect aerial populations of RWA.  The suction trap was located near Briggsdale, CO., adjacent to the field site used for collecting RWA in the first procedure.  Small insects flying over the suction trap are pulled down to the base of the tube where a collection jar containing a clean kim-wipe was located.  Russian wheat aphid samples were collected on an hourly basis on June 10, 19, 30 and July 10, 1992.  In 1994, suction trapped RWA were collected on an hourly basis on June 18, 19 and 23.  Suction trapped RWA were returned to the laboratory and frozen for later lipid analysis.

 The third method used to collect RWA samples involved the use of a Hiller UH12D helicopter.  Helicopter sampling was conducted in June of 1994.  The helicopter was equipped with two identical insect collectors mounted on either side (Hendrie et al. 1985, Isard et al. 1990).  The collector was a cylindrical expansion chamber, 0.81 M diameter and 1.8 M long with and inner conical stainless steel sieve.  The inner conical sieve concentrated the insects into a collection chamber at the rear of the expansion chamber.  Each collection chamber had 24 individual compartments.  The compartments were opened and closed by the helicopter operator, allowing for the separation of catches according to altitude.  Aerial sampling took place over wheat fields near Greeley, Colorado at altitudes ranging from ground level to 500 meters above ground level.  At the end of each flight the collection chambers were removed and each compartment was carefully inspected for RWA samples.  Russian wheat aphid samples were placed in four ml vials, labeled according to the altitude trapped and then frozen for subsequent lipid analysis.
 
 

Lipid Extraction and Analysis

 Methyl ester standards, tetradecyl acetate, and 14% boron trifluoride in methanol (14% BF3 in MeOH) were purchased from the Sigma Chemical Company, St. Louis, MO.  All other materials were of reagent grade and were distilled to insure purity.  Pasteur pipets, four ml vials, and filtering materials were cleaned initially with dichloromethane.  The four ml vials used for methanolysis were further cleaned by heating the vial at 100 C for one hour with 1 ml of the methanolysis reagent (14% BF3 in MeOH).

 Methods similar to those of Bligh and Dyer (1959) and Morrison and Smith (1964) were used to extract individual RWA lipids.  An individual aphid was placed in a clean four ml vial with a Teflon-lined screw cap.  One milliliter of 2:1 chloroform:methanol was added to the vial.  The aphid was macerated with a glass rod, and the extract was filtered and evaporated to apparent dryness with a nitrogen stream.

 Dried lipid extracts were methylated with 0.5 ml 14% BF3 in MeOH (60 C, one hour).  The fatty acid methyl esters were isolated by adding one ml of hexane and one ml of water to the vial.  The vial was shaken and the lower water layer was discarded.  The hexane extract was then washed twice with water (or until neutral as indicated by pH indicator paper), filtered, and concentrated with a nitrogen stream.

 The derivatives were analyzed with a Hewlett-Packard 5890 gas-liquid chromatograph (GLC) equipped with a Carbowax column (30 m x 0.25 mm, 0.25 um film thickness) (Alltech Associates, Dearfield, IL.).  The column was temperature programmed from 60 C to 240 C at 10 C/min (one min delay) for a total run time of 30 minutes.  Lipid classes were identified by comparing their retention times to those of known methyl ester standards.  Amounts of fatty acid methyl esters were determined from peak areas by using an internal standard (tetradecyl acetate).
 
 

Statistical Analyses

 Fatty acyl components isolated from RWA were subjected to analysis of variance on  Microsoft Excel, Analysis Tools, version 4.0. (Microsoft Corporation 16011 NE 36th Way, Box 97017, Redmond WA. 98073-9717).  Experiments in which ANOVA yielded significant results were subjected to Newman-Keuls multiple range test allowing for unequally sized data sets (Zar 1984).
 

Results

 A total of 75 individual RWA collected from the ground-based suction trap during June and July of 1992 were analyzed for lipid content (Fig. 3.1).  The lipid profiles of RWA over this period of time remained fairly constant with the exception of the tetradecanoate component in the population trapped on June 19, 1992.  The predominant fatty acyl component in populations of RWA trapped between June 10 and July 10, 1992 was tetradecanoate (Fig. 3.1).  Lipid analysis comparing the time of day individual RWA were trapped: 7-11 A.M., 11-3 P.M. and 3-6 P.M., yielded no significant differences (Fig. 3.2).

 Russian wheat aphids collected from wheat tillers had significantly greater amounts of fatty acyl components than RWA that were suction trapped (Fig. 3.3).  Further analysis of field-collected RWA demonstrated that samples of RWA of an unknown age (or flight history) had significantly greater amounts of the 12 and 14 carbon fatty acyl components than did field-collected apterous RWA that were reared to alate adults (12-24 hours old) in petri dishes (Fig. 3.4).  Both alate RWA collected from the field and field-collected RWA nymphs that were reared in petri dishes contained significantly greater amounts of the 12 and 14 carbon fatty acyl components, as compared to suction trapped aphids (Fig. 3.4).   Lipid analysis of Russian wheat aphids collected during the summer of 1994 were similar to the lipid profiles of RWA collected in 1992, with the following exceptions.  Field collected RWA that were returned to the lab and reared in petri dishes (less than 12 hours old) had significantly greater amounts (p=0.05) of dodecanoate, tetradecanoate, (Z,Z)-9,12-octadecadienoate and (Z,Z,Z)-9,12,15-octadecatrienoate components, as compared to field collected samples of an unknown age or prior flight history (Fig. 3.5).  The tetradecanoate component of both field-collected RWA populations (lab-reared and unknown life history) was significantly greater than the tetradecanoate component in suction trapped RWA (Fig. 3.5).

 Russian wheat aphid populations that were collected by helicopter mounted traps contained the greatest amount of the 14 carbon fatty acyl component (Fig. 3.5).  The amount of the tetradecanoate component in helicopter trapped RWA was significantly greater than that of both suction-trapped RWA and  field-collected RWA populations of an unknown life history (p=0.05).  The tetradecanoate component of helicopter trapped RWA was not significantly different from field-collected RWA that were less than 12 hours old.  Of the remaining fatty acyl components, suction trapped RWA consistently had the least amount present (Fig. 3.5).
 
 

 Discussion

 Earlier experiments in this thesis indicate that alate RWA utilize lipid reserves, specifically the tetradecanoate component, as a fuel for flight energetics.  In addition, the tetradecanoate content in RWA rises to a maximum approximately 10 hours after eclosion, which is correlated with the greatest propensity for flight.  This relationship between flight duration and lipid reserves has also been reported in Aphis fabae (Cockbain 1961) and Rhopalosiphum maidis (Liquido and Irwin 1986).  The results of the present study provide further evidence confirming the previously held hypothesis that the RWA uses the fatty acyl component tetradecanoate as a principle flight fuel.

 Lipid analysis of RWA that were trapped with a ground-based suction trap consistently demonstrated that these aphids had the lowest lipid reserves.  As discussed in chapter two, RWA lipid reserves, primarily tetradecanoate, decrease as a function of age and prior flight history.  The suction trap data therefore indicates that these aphids are either older aphids, aphids that have exhausted their lipid reserves in a long flight, or both.

 In the 1992 study, field-collected RWA of an unknown life history had significantly greater amounts of the dodecanoate and tetradecanoate components than did aphids that were collected as nymphs in the field and reared to adults within a petri dish.  Confining the aphids within petri dishes restricted flight so any reduction in lipid reserves is most likely related to changes that occur only as a function of age.  Aphids confined to petri dishes were 12-24 hours old, an age that is characterized by reduced lipid levels (see chapter two).  This data indicated that randomly collected RWA that demonstrate high lipid levels are younger alate aphids (less than 12 hours old).  In the 1994 study, field-collected, laboratory reared RWA were less than 12 hours old and this group of aphids had greater amounts of the 12 and 14 carbon fatty acyl components than did randomly collected field aphids of an unknown life history.  This suggests that during the field collection in 1994, most alates were older than 10 hours post-eclosion.

 Perhaps one of the most interesting results from the field study was the lipid analysis of RWA trapped by helicopter.  The lipid profile of the 26 individual RWA trapped at altitudes ranging from one meter above ground to 500 meters above ground was not correlated with the total lipid levels present.  Suction trap data provided reason to expect that those aphids trapped at the highest altitudes would have the lowest lipid levels.  However, the data from helicopter trapped RWA indicated otherwise, and the mean lipid content for this group was actually the highest.

 Observations by Isard et al. 1990, indicated that the radiative heating of ground surfaces during the day create upward convection currents that act to enhance the upward movement of aphids.  In addition, prior to flight, aphids show a strong phototactic response to light of short wavelengths and seek the highest point before initiating flight (Kennedy et al. 1961).  Kennedy and Booth 1963, demonstrated that A. fabae continues this positive phototaxis to ultraviolet-blue light for up to four hours after takeoff.  Surfaces reflecting yellow and orange light then begin to act as a behavioral phototactic stimuli to initiate downward flight (Kennedy et al. 1961, Kring 1969, Kring 1990).

 Without flight energetic experiments, it is not clear to what extent the ascent and descent of aphids is due to active flying or merely being transported by ascending and descending air currents (Johnson 1969, Kennedy 1975).  There has been some speculation that aphids may cease flying and simply fall back towards the ground (Johnson 1969).  A carefully constructed study on the sinking speeds of A. fabae by Thomas et al. (1977), indicated that aphids do not just fall back to the ground.  Thomas et al. (1977), concluded that A. fabae "responds to surface colors and actively flies downward, abdomen first, at speeds up to more than 0.7 M/S".

 Lipid analysis of populations of RWA trapped by helicopter at various altitudes suggests that the RWA does not use its lipid reserves during vertical ascent.   Results indicate the RWA is assisted during vertical ascent by upward moving air currents and then uses lipid reserves, primarily the tetradecanoate component, to actively fly downward.  This conclusion is supported by data demonstrating that aerial populations trapped by helicopter had the most lipid reserves, while suction-trapped aphids (at 8.3 meters) had the least, and lipid levels of randomly collected field populations fell in-between helicopter-collected and suction-trapped RWA samples.

 Results of the present study indicate that determining lipid levels in the RWA can provide insights into the aphid's stage of development and or prior flight history.   An analysis of RWA randomly collected from field sites would be a good indicator of the prior flight history of those aphids.   Russian wheat aphids with low lipid levels would indicate that the samples are older aphids that may have dispersed into the area from other localities.   Lipid levels of the RWA alone would not indicate probable dispersal sites but could be correlated with previous weather patterns to make inferences concerning a populations origin (Isard et al. 1990).
 

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