Ecological interaction among natural enemies and its consequences for biological control
One goal in pest control is to develop methods that suppress pests without harming other organisms. With this in mind, an ideal method would be to augment nontarget organisms to control a pest. This can be accomplished by augmenting natural enemies. However, augmentative biological control is often difficult and complicated. For example, when several species of natural enemies attack the same pest population, their action may combine to improve biocontrol. On the other hand, examples also exist of natural enemies disrupting each other and decreasing pest control. I review recent literature on arthropod systems and find more cases of improved biocontrol with multiple natural enemy species (Chapter 1). The behavior of individual arthropods can sometimes explain whether different natural enemies are compatible for pest control. I present empirical data on the foraging behaviors of several natural enemies in western Washington pea fields (Chapter 2). These natural enemies consume the primary pest in the system, the pea aphid (Acyrthosiphon pisum), but they also attack each other ("intraguild predation"). The intraguild predators include lacewings (Chrysoperla carnea) and ladybird beetles (Coccinellidae), while the intraguild prey include syrphid flies (Syrphidae) and parasitoid wasps (Aphidius spp.). I could not find a statistically significant difference in encounter rate with pea aphids of intraguild predators compared to intraguild prey. If encounter rates reflect the exploitative ability of the natural enemies in the system, then theory predicts that intraguild predation will not disrupt biocontrol. Field experimentation suggests that intraguild predation does not disrupt biocontrol of the pea aphid (Chapter 3). I conducted several field experiments in which I augmented populations of lacewings or ladybird beetles. If lacewings or ladybird beetles disrupt biocontrol, then pea aphid populations should increase in predator-treated plots. Instead, in four out of five experiments, predator augmentation was associated with statistically insignificant decreases in pea aphid populations. Thus, while intraguild predation does not disrupt biological control of the pea aphid, predator augmentation did not substantially improve aphid control. The effect of predator augmentation was dwarfed by unmanipulated large-scale factors such as weather and site location.
- Biology