Behavior of Parasitoids and their Hosts

 Introduction

 	In the world of parasitoids and hosts, behavior has evolved to a very intricate level. I am going to 
be looking at some specific examples of different species behavior, and the interaction of the host and the 
parasitoid.  All of the parasitoids I will be investigating are of the order hymenoptera, with one exception.  
The hosts of these different Hymenopterans range from Homopterans to Lepidopterans and even other 
members of the Hymenopteran order.  The behavior of the parasitoids, in some cases, depends on the 
physiological state, i.e. the age of the parasitoid, or the age of, or particular instar of the host.  The 
initiation of oviposition by certain parasitoids is dependent on various cues given by the host, or a 
physiological agent within the biology of the parasitoid.  Physiology plays a definite role in the behavior of 
the parasitoid, mostly the age.  Population of the host is another factor that influences the behavior of the 
parasitoid, along with plant quality that some of the hosts feed, and live on.

 	A repeating factor in the oviposition behavior by a parasitoid is previous experience by the 
parasitoid.  Experience of past attacks and ovipositions greatly increases the chances for later oviposition.  
I will explore this behavior later in the article.  Host behavior also plays a large role in whether or not the 
host is  suitable for rearing young on.  Defense behavior by the host, or passive behavior when being 
attacked can either stimulate or turn off some parasitoid species. 
 
 	Host size is a definite cue to initiate a parasitoid to attack and oviposite.  Host size is a factor of 
host suitability.  A large host means that the larvae of the parasitoid will have more resources than that of a 
larvae developing on a smaller host.  Therefore it is safe to say that the behavior of the parasitoid should 
reflect this situation.  This is not always the case, some parasitoid larvae have flexible growth which 
allows the use of a wide selection of host sizes.  This facet of behavior will be investigated later in the 
article.  
 
 Discussion
 
 	First off, I would like to look at a species of parasitoid that uses past experience of oviposition to 
govern the behavior of future interaction with a host.  The parasitoid Brachymeria Intermedia is a 
endoparasitoid of Lymantria dispar.	 It is documented that this type of parasitoid will parasitize a host 
more readily if she has had earlier oviposition experience(Kerguelen & R. T. Carde, 1994).  It seems that 
the number of females that accept a host increase gradually with the number of previous oviposition 
experiences, and with the amount of experience with handling a host.  It seems, in a study done, that a 
single antennal contact with a host will increase the possibility of interest in the host by this parasitoid.  
Antennal contact with a pupa of the gypsy moth is one of the first acceptance behavior which initiates a 
very gradual process.  It is probable that naive wasps first learn the host odour with that first contact of 
antennae to the pupa.  After the first contact they are more readily attracted to the host in later encounters 
when exposed to the host odour, so this is obviously the first behavior of many in becoming an 
experienced wasp.  

 	Behavior of the host can also be modified by experience in many  ways, but most of the behavioral 
changes happen in the parasitoid.  Learning is demonstrated to account for the behavioral changes that 
happen through experience.  In an experienced mature female of B. Intermedia encountering a perfect host, 
the first behavior to happen is a very thorough investigation by the wasp's antennae.  This eventually leads 
to oviposition.  Here is a standard ethogram of the behavior of an experienced wasp encountering a 
suitable host:
 
 From the ethogram, it shows that the behavior by the wasp can change in either direction if it feels that the 
host will not do.  It seems that the process of a naive wasp becoming an experienced wasp is not a quick 
process that just happens.  It is a very gradual process.  From the first acceptance behavior, which is 
antennae contact, to ovipostion is gradual.  An experiment done by V. Kerguelen and R.T. Carde showed 
this process of behaviors:
  
 	This graph shows the percentages of females that accept, reject, and show no response after a 
series of exposures to a pupa.  For the naive females the majority shoed no response.  The females that 
just touched their antennae to the pupa had a high rejection rate.  The females that were allowed to drum, 
including the previous behavior showed the percent of the females that accepted to be a touch higher.  The 
females that were allowed to insert ovipositor, but not oviposite, including previous behavior showed a 10 
percent difference in acceptance behavior to those that rejected.  the females that actually got to oviposite 
including all the previous behaviors showed a large acceptance percentage, and the females that had a 
normal ovisposition sequence showed the largest acceptance percentage of the host.  This proves that the 
process of a naive female wasp to becoming an experienced female is a gradual process.  
 	
 A Look at Instar Preference and How Age Affects Behavior.  
 	
 	The parasitoid Lysiphlebus cardui is a parasitoid of the Homoptera Aphis fabae cirsiiacanthoidis.  
The behavior and host instar preference is a function of the age of the wasp.  It seems that young 
parasitoids of a day old or less seem to parasitize aphids of second and third instars, where as older 
parasitoids don't even show instar preference.  This shows that oviposition behavior of L. cardui is 
governed by the age of the wasp.

 	Parasitoid insects face many dilemmas when searching out and finding a host, for example, which 
type of hosts to attack and what behavior is exhibited by the host to initiate oviposition.  Also what type of 
defense reaction the host gives to deter oviposition, and how to avoid superparasitism.  

 	The defense reaction of Aphis fabae cirsiiacanthoidis is to kick away a marauding wasp, and only 
fourth instar and adult aphids do this.  Young parasitoids of L. cardui almost completely ignore aphid 
colonies.  Even on more than one encounter, no attack or oviposition behavior is exhibited.  This behavior 
differs for older females which normally attack any aphid they encounter.  Here is a graph that illustrates 
these behaviors:
  
 The dark cross hatching represents the number of wasps who oviposited on at least one aphid.  The black 
bars indicate the number of females that attacked but did not oviposite on at least one aphid, and the bars 
with light and no shading indicate females that encountered at least one aphid, or ignored aphids 
completely.  As you can see the young wasps have a very low oviposition percentage, and the older wasps 
have a very high oviposition percentage.

 	Now looking at the instar preferences of the different ages of wasps.  Young females lay more 
eggs into second instar and third instar aphids and not so many in fourth instar and adults.  Older females 
of the wasp are not as picky but do prefer younger instars as well.  The reason for this is most likely the 
defense reaction that the fourth instar and adult aphids exhibit.  Kicks are the main defense.  A strange 
paradox is that attacks from an older parasitoid produce less defense reactions from fourth instar and adult 
aphids than attacks by young parasitoids.  The reason for this is not well documented.  

 	Overall the fourth instar and adults are less attractive to both old and young wasps.  It is still not 
clear whether fourth instar and adults are of inferior quality or that younger instars are just an easier victim.  
Obviously the way in which eggs are distributed through an aphid colony depends on the age of the 
parasitoid, the instar class of the aphid, the defense behavior of the aphid, and the search behavior of the 
wasp.  
 
 Importance of Host Body Size.
 
 	This is the only species of parasitoid that I looked at that is not a Hymenoptera.  The Conopid fly is 
in the order Diptera.  It parasitizes the bumblebee Bombus terrestris.  Host selection by this parasitoid is 
directly related to host body size.  Bees that are parasitized by conopid parasitoids are on the average larger 
bodied than unparasitized bees. 

 	The problem is that there is no definite research that has proven that the reason for the number of 
larger-bodied bees being parasitized is on average greater than smaller bees is due to an active choice by 
the conopid flies, or is it due to the fact that larger bumblebees are slower and easier to target.  The 
behavior of the conopid fly is obviously a factor in the choosing of large-bodied bees(if it is actually a 
choice).
 
 Host Population and how it affects parasitoid behavior.
 
 	One unusual behavior of some parasitoids is the failure to persist when a host is abundant.  If there 
is an overpopulated plant infested with aphids, a parasitoid is wise not to parasitize any of the aphids.  
This is due to the high mortality rate of aphids on an overpopulated plant.  This behavior has been 
documented in some species of hymenoptera.

 	The wasp Aphidius ervi is one wasp that does parasitize aphids that are low in numbers.  This 
behavior is very beneficial due to the fact that there is a higher mortality rate within populations of great 
numbers.  If the parastized aphid eventually dies, it is safe to say that the parasitoid larva will die as well.

 	An experiment was done showing the number of attacks on  a different number of aphids in a 
single plant:
  
 This graph shows three different plants with different populations on each.  As illustrated, there is a larger 
amount of attacks on the plant with only two aphids, the number of attacks decreased when there were 
eight aphids, and the number of attacks decreased dramatically with thirty-two aphids on one plant.  

 	A. ervi is obviously attracted to low aphid populations.  A. ervi is very selective of what host it 
chooses, however what governs the selection is not the individual aphid, but the number of individuals 
within a particular colony.  It is interesting that some parasitoids are very selective about the individual 
while others take a more broad approach and look at the entire population.
 
 	It is documented that if A. ervi had to lay eggs in a large population of aphids that it would go 
extinct.  This is due to the high mortality of parasitized aphids within a large population.  This means that 
A. ervi as a biological control of  the pea aphid is impossible.  
 
 How Host Behavior Influences Parasitoids and Ways a Parasitoid Finds its Host.
 
 	A successful host-parasitoid association is made up of host habitat location, host location, host 
acceptance, host suitability and host regulation.  The hosts behaviors are mostly avoidance and then 
defense. The defense behaviors range from biting to aiming fluids at the parasitoids head.  One way that 
the parasitoid has overcome these defenses is to paralyze the host before laying an egg.  Other examples of 
counter behaviors by parasitoids include climbing down silk threads produced by fleeing host, and  
ovipositing on specific soft spots of armored hosts . 

 	The wasp Venturia Canescens use a number of methods to find and attack a host.  When attacking 
smaller hosts, it first locates them by their antennae.  Once found it holds them still and oviposits an egg.  
Larger hosts, after detected, are pursued and attacked very rapidly.  The whole sequence of events, from 
detection to oviposition takes only a few seconds.  In some cases the host Corcyra cephalonica in instars 
one through four are immersed in the medium in which they live. Larvae in the fifth instar frequently crawl 
on the surface during the "wandering phase".  The parasitoid can exploit both instars.  It searches 
vibrostatically, and by probing the substrate with it's ovipositor.  C. cephalonica is a very large host 
compared to the wasp.  V. canescens frequently ignored fifth instar C. cephalonica after antennal contact, 
but when antennal contact is made with a much smaller host species Plodia interpunctella it is immediately 
attacked.  This preference for P. interpunctella by V. canescens is a behavioral interaction.  It is preferred 
because C. cephalonica is more aggressive and harder to parasitize.  C. cephalonica's defense behavior is 
savage head and tail flicking and wriggling back and forth, where as P. interpunctella has more avoidance 
behavior such as crawling rapidly and burying itself.  
 
 Unusual Foraging Behaviors.
 
 	There is one specific wasp with quite interesting foraging behavior.  It is the Pauesia unilachni.  It 
is a parasitoid of the grey pine aphid, Schizolachnus pineti.  The grey pine aphid feeds off the needles of 
Scots pines.  It seems that the female wasps have a dual movement system when foraging in the pine.  The 
females leave their colony usually by foot and search needles that are close by.  This way it insures itself to 
first find nearby colonies of S. pineti.  If the females don't find any S. pineti colonies on foot they switch 
their behavior and search needles by taking to the air, covering very large distances.

 	Having an efficient searching strategy is of crucial importance for optimizing the amount of 
offspring a parasitoid has.  The first step in a prosperous search is the location of host plants or hosts.  
Host location can be classified into three categories: Stimuli directly associated with the host, stimuli 
indirectly associated with the host, and stimuli directly associated with the host plant or microhabitat.  

 	Kairomones are often used by parasitoids which guide them directly to the host.  Most of these 
Kairomones are only useful at short to medium distances.  There are some long-range attractant 
kairomones, an example of which is the parasitoid Aphidius rosae. This aphid parasitoid is able to detect 
colonies of its host at a distance of 20cm.  The females use aphid born cues to orient themselves.  Some 
parasitoids are unable to use chemical cues so they may search a host plant at random. For example, 
searching for a concealed host may be very difficult since the host is protected by the plant in which it 
lives.  In some plant tissues it is hard to distinguish host containing tissues from non-host containing 
tissues.  In this case random probing with the ovipositor is done.  Another example of the random 
searching is the females of Agathis spec.  It parasitizes the larvae of the moth Greya subelba.  This larvae 
feed on the plant Lomatius dissectum.  The parasitoid is not able to distinguish between an invested plant 
and an uninfested plant, so it randomly searches the host plants.  

 	As seen with the parasitoids, their behavior has evolved to benefit the survival of their species.  
Foraging behavior and oviposition behavior have been manipulated over time to give the parasitoid the 
most efficient and successful results of parasitizing a particular host.  These complex behaviors that I have 
looked at only scratch the surface of a larger world. Each parasitoid has distinct behavior that is unique to 
itself.  The different strategies of foraging to optimize time and energy is of staggering complexity and 
beauty.  The cues that parasitoids home in on range from a complex mixture of kairomones that a particular 
host gives off to randomly searching for a host.  

 	Host behavior does not seem to be as complex when it comes to the relationship with a parasitoid.  
All of the behavior exhibited by the variety of hosts I looked at were defense and avoidance behaviors.  
Evasive actions seem to be the most common, but a few species of hosts do have some intriguing defense 
reactions, for example the body fluids that are excreted from a host being attacked by a parasitoid.  
 
 	Overall the behavior exhibited by the parasitoids is fascinating.  With all the factors that come into 
play for any species to survive, the parasitoid is one type of insect that has survival down to an art.  There 
are many benefits to studying biology and behavior of these incredible insects.  One of the main reasons 
for research is the possibility for the biological control of many pest species of Lepidoptra and Homoptera.  

 References   
   
 Schmid-Hempel R, Schmid-Hempel P, (1996) Field evidence that host selection by Conopid parasitoids 
is related to body size. Oecologia Vol: 107 77-78
 
 Harvey J, Thompson DJ, (1995) Host behavior and its influence on foraging and acceptance by solitary 
parasitoid wasps. Entomophaga Vol: 40 193-210
 
 Ives A R, Settle W,(1996) The failure of a parasitoid to persist with a superabundant host: the importance 
of the numeric response. OIKOS Vol 75: 269-278
 
 Tang Y. Q. Yokomi R. (1996) Effect of parasitism by Aphelinus spiracolae on development and 
reprodution of Spirea Aphid. Environmental Ent. Vol 25:no. 3 703-706
 
 Mackauer M, (1996) Sexual size dimorphism in solitary parasitoid wasps: influence of host quality. 
OIKOS Vol 76: 265-272
 
 Rosenheim J, Mangel M, (1994) Patch leaving rules for parasitoids with imperfect host discrimination. 
Ecological Ent. Vol 19: 374-380
 
 Harvey J A, Harvey I F, Thompson D, (1994) Flexable larval growth allows use of a range of host sizes 
by a parasitoid wasp. Ecology Vol 75: 1420-1428
 
 Kerguelen V. Carde R.T. (1996) Increased accptance in experienced females of the parasitoid 
Brachymeria intermedia: which types of oviposition behaviour contribute to experience. Entomologia Exp. 
et App. Vol 78: 95-103
 
 Weisser W. (1994) Age-dependant foraging behavior and host instar preference of the aphid parasitoid 
Lysiphlebus cardui. Entomol. exp. appl. Vol 70: 1-10
 
 Volkl W, Kraus W. (1996) Foraging behaviour and resorce utilization of the aphid parasitoid Pauesia 
unilachni: adaptation to host distribution and mortality risks. Entomol. exp. appl. Vol. 79: 101-109
 
 Brodeur J, Geervliet J.B.F. Vet L. (1996) The role of host species, age and defense behaviour on 
ovipositional decisions in a solitary specialist and gregarious generalist parasitoid. Entomol. exp. appl. 
Vol. 81: 125-132
 
 Harvey JA, Thompson DJ (1995) Host behavior and its influence on foraging and acceptance by the 
solitary parasitoid wasp, Venturia canescens. Entomophaga Vol. 40: Iss. 2, 193-210
 
 Walter GH (1993) Oviposition behavior of diphagous parasitoids- A case of intersexual resource 
partitioning. Behaviour Vol 124: 73-87