ABSTRACT Biological control of spider mites has become a topic of much interest, since these pests have developed resistances to many pesticides and miticides. Understanding the behavior of spider mites as well as the predaceous mites is significant for successful biological control. Dispersal behavior of different predatory mites will affect how well they control different types of infestations. Use of generalist or specialist predatory mites results in different types of control (Grafton-Cardwell 1997). Generalist mites move around individual plants more, whereas specialists cover an over all broader area. Herbivore-induced plant volatiles have effects on both spider and predacious mites. Different species elicit different volatiles from the same plant, allowing for specific recognition (Margolies 1997). Various behaviors occur in response to conspecific and heterospecific competitors. There are certain advantages or disadvantages in selecting a plant already inhabited by conspecific or heterospecific competetors. Intra- and inter specific predation occurs between predatory mites (Croft 1998). This needs to be considered before release of predatory mites. Mites can be either feeding, resting, or moving. The allocation of these behaviors is significant to the interactions of plant, prey, and predator. The amount of time spent doing each activity by spider and predacious mites of various ages is discussed.

INTRODUCTION

There are a variety of species of spider mites, but they are not readily distinguishable in the field. The life cycles, diets, and predators of the various species are virtually identical. Spider mites are in the class Arachnida and are closely related to ticks. Mites have a six legged nymph stage and an eight legged adult stage. They have a one-sectioned body and lack compound eyes. Mites lack chewing mouth parts, but use chelicerae to suck out cell contents. Spider mites are serious pests on over 30 types of economically significant crops. Leaf damage occurs from spider mites because they remove chlorophyll, water, and nutrients from leaf cells. Infestations are recognizable by spotting of plants and a network of webbing on the underside of the leaves. Heavy infestations will kill a plant. Infestations often spread to crops from the preferred hosts, weeds and certain flowers, that are nearby. Clearing these plants from around crops can reduce infestations. This method is effective, but is simply not enough. After more than twenty-five years of pesticide and miticide use, spider mites have developed a strong resistance to them. Farmers are turning to biological control methods. Predaceous mites can drastically reduce spider mite infestations. There are many behavioral aspects that need to be considered in both spider mites and predacious mites. Recognition of these behaviors can increase the success of biological control.

DISCUSSION

Dispersal behavior

Within- and between-plant dispersal of predacious mites is directly related to how well they control infestations. Two common predacious mites are Neoseiulus californicus and Neoseiulus fallacis. N. californicus has greater within-plant dispersal than N. fallacis. N. fallacis moves sooner at low prey densities and disperses more between plants (Pratt 1998). Movement based on prey densities is a significant factor in the long term effectiveness of biological control. The different behaviors of these mites is due to their feeding style. N. fallacis is a specialist predator, eating only spider mites. It is necessary for N. fallacis to move around more because it must find prey and cannot live off other food sources. This species is dependent on highly aggregated colonies of prey. N. californicus, on the other hand, is a generalist predator, and can feed off of pollen and leaf sap if prey is not available. This would account for N. californicus not moving as far away from a release site as N. fallacis would.

These two species, one generalist, the other specialist, offer different types of biological control. N. fallacis could provide better control for heavy infestations, but would disperse too much after prey is reduced for long term control. N. fallacis has higher rates of very local movement in prey patches or individual leaves (Pratt 1998). This species is more active than many others. N. fallacis moves less often between prey patches, but is more active within them (Pratt 1998). N. californicus would provide more long term control, as they can live off of other food sources when prey is scarce, therefore dispersing less. N. californicus has a more even control pattern. All the leaves on a plant will have spider mites reduced to about the same degree when treated with N. californicus, as it searches all areas of a plant equally. N. californicus will not be as effective on highly aggregated prey mites. This species moves more often between prey patches, but stays for shorter periods. N. fallacis has uneven control patterns. A large patch of spider mites may be eliminated on one part of the plant, but another patch may go untouched.

Egg dispersal is another significant factor. N. californicus disperses its eggs more than N. fallacis (Pratt 1998). The protonymphs and deutonymphs of N. californicus are more active than those of N. fallacis. Predatory mites with larvae that feed more tend to grow into nymphs that feed less. These nymphs may be more specialized predators of spider mites.

Specialist predators have higher rates of reproduction, better searching ability, and reduce prey populations faster (Grafton-Cardwell 1997). Generalists are more common in subtropical tree crops and maintain themselves longer. Using gereralist species may only require one release, whereas specialist species may require several releases. If the pest infestations are more dispersed, a generalist predator would be more effective. If the infestations are more aggregated, a specialist predator would be more effective. Depending on the type of infestation, one might want specialists or generalists or both.

Responses to herbivore-induced plant volatiles

Plants produce volatiles when under herbivore attack. These odors are recognizable to predators as well as other herbivores. Herbivores can have two responses to these volatiles. They can take it to mean a food source with already weakened defenses, or a food source to avoid because of competition. Spider mites can distinguish among the odors of clean plants, plants with thrips, and plants with conspecifics (Pallini 1996). Spider mites have a slight preference for plants that are already infested with conspecifics. On the other hand, they have a strong avoidance reaction to plants infested with heterospecific thrips (Pallini 1996). The plant volatiles produced from thrip infestations are chemically different from the volatile produced from spider mite infestations. The reasons for avoidance of thrip infested plants is simple. When thrips aren’t eating leaf tissue, they like to eat spider mites. This behavior avoids both competition and predation. This behavior has logical evolutionary pressure.

The attraction to plants with conspecifics is not as strong as the avoidance reaction to thrips. In olfactory tests, the volatiles of conspecifics was selected only slightly more than a clean plant. It has been suggested that intrinsic growth rates increase with spider mite densities. This would make it beneficial to select a plant with conspecifics. Another possibility has to do with the webbing that allows for locomotion and defense from some predators. Not having to spin this web is beneficial in terms of energy use. However, some predators have better mobility on spider mite webbing. The volatiles of conspecifics may also be used to locate empty sites within the plant (Pallini 1996).

This intraguild predation leads to various population dynamics. One herbivore could be excluded, or coexistence could occur. In a complex trophic relationship the success of biological control could be affected.

Predatory mites also respond to herbivore-induced plant volatiles. This chemical cue tells them where prey are. The intensity of the volatiles is directly related the the number of potential prey in an area (Pallini 1996). Energy will be conserved as predation is increased and movement is decreased. Predatory mites also avoid plants with conspecifics.

Predation within predators

Some generalist predators will prey on other predatory mites when other food sources are scarce (Croft 1998). Certain predatory mites are slower and less evasive than others, leaving them more susceptible to predation. Some species of predatory mites have been known to eat the young of other species, as well as the young of their own. Some predatory mites, in cases of extreme food shortage, will lay all their remaining eggs and eat them (Croft 1998). This behavior can increase chances of individual behavior. Intra- and inter-specific predation should be considered when deciding what types of predatory mites to use on infestations.

Time spent feeding, moving, and resting

A tritrophic interaction between plant, predator and prey needs to be recognized for biological control. Times spent feeding, moving, and resting can be compared between healthy and spider mite-damaged leaves, between immature and adult spider mites, and between immature and adult predatory mites. Juvenile spider mites spend more total time stationary than moving, but spend more time moving when on damaged leaves (Bancroft 1996). Juveniles spend more time moving on damaged leaves because of a reduced food source. Juveniles have relatively poor dispersal capabilities compared to the Adults. Adult spider mites spend more total time moving, regardless of leaf damage (Bancroft 1996). This is the stage in the life cycle when adults are dispersing. While on clean leaves, the adult spends almost no time resting. When the adults become older, the feeding bouts decrease, the frequency of occurrence increases, but total time spent feeding is increased (Bancroft 1996). Moving bouts become shorter on clean leaves, but remain the same on damaged leaves. This would reflect a movement away from depleted food sources to new ones. A similar response occurs in the presence of predatory mites. Spider mites move more and feed less if a predatory mite is near.

The oviposition life stage occurs as the female spider mites age. Oviposition requires approximately one minute of resting. On clean leaves, a network of webbing is often formed on the midrib before egg laying.

Juvenile predatory mites, like the spider mites, move less than the adults. Juveniles spend more time resting than feeding or moving, and the adults spend more time moving than feeding or resting (Bancroft 1996). Juveniles have lower energy requirements and feed fewer times than adults do. It takes a juvenile four times as long to eat a prey egg as for an adult. Predatory mites spend more time resting and less time feeding than spider mites. Adult predatory mites rested more frequently, but for shorter durations than the juveniles (Bancroft 1996). It should be noted that in the presence of abundant prey, time is not the limiting factor on consumption. A predatory mite could consume twenty times more than it does with the time allowed. The gut absorption rate is what limits the rate of consumption (Bancroft 1996).