Enemy-Risk Effects in Parasitoid-Exposed Diamondback Moth Larvae: Potential Mediation of the Interaction by Host Plants

SIMPLE SUMMARY: While consumption by predators directly reduces prey populations, some studies have shown that even the mere threat of predation can negatively impact the health and fitness of prey. This is referred to as the enemy-risk effect. We investigated enemy-risk effects in caterpillars (larval stage of the diamondback moth, Plutella xylostella) feeding on two different host plants in the presence and absence of the parasitoid wasp, Diadegma insulare. We found that the threat of attack can slow down caterpillar development and produce smaller adult moths. These physiological changes coincided with a change in caterpillar behavior to feed on older foliage, and the host plant species appeared to influence the severity of the enemy-risk effects. ABSTRACT: Enemy-risk effects (i.e., non-consumptive effects) describe the non-lethal fitness costs incurred by animals when they perceive a risk of predation. These effects can result from fear-associated changes in behavior and physiology. Diamondback moth larvae (Plutella xylostella) are known to violently wriggle backwards and drop from their host plants, usually suspending themselves with a silk thread, when threatened by predators and parasitoids. Here, we investigated the developmental costs associated with this behavior when larvae were exposed to its specialist parasitoid wasp (Diadegma insulare). Additionally, the structural and chemical properties of plants are well-known to influence predation and parasitism rates of herbivorous insects. Yet, few studies have examined the influence of plants on enemy-risk effects. Therefore, we examined the developmental costs associated with parasitism risk on two host plants. Diamondback moth larvae were placed on either cabbage or Virginia pepperweed plants and exposed to gravid parasitoids with truncated ovipositors, which prevented piercing of the host cuticle without affecting host searching and attacking behaviors. On Virginia pepperweed, risk of parasitism resulted in reduced larval weight gain, longer development time, and smaller adult size compared to larvae that were not exposed to parasitoids. However, on cabbage, parasitoid exposure prolonged development time but had no significant effects on larval weight gain and adult size. On both plants, parasitoid-exposed larvae were found feeding on older foliage than younger foliage. Our findings demonstrate that the enemy-escape behavior of diamondback moths has developmental costs and that plants may mediate the intensity of these enemy-risk effects.