Olfactory Cues Mediate Multitrophic Interactions Among Cucumber Plants, Cucumber Beetle Larvae and Entomopathogenic Nematodes

Olfactory cues mediate a wide variety of ecological interactions among organisms at different trophic levels. There is abundant evidence that these cues play critical roles for organisms foraging for resources and defending against potential attackers. Some of the best-studied examples include plants producing volatile organic compounds to defend themselves against herbivores and natural enemies using prey-associated odors while hunting. While much of this research has focused on aboveground systems, there is growing recognition that olfactory cues also facilitate multitrophic interactions among soil-dwelling organisms. The overall purpose of this thesis was to examine how olfactory cues from plants and natural enemies guide the foraging decisions of herbivores and their natural enemies, focusing on a belowground tritrophic system. First, I review the literature to examine how plant-associated microorganisms alter plant phenotypes to influence herbivore foraging behavior. Next, I investigate the roles of herbivore-induced plant volatiles (HIPVs) from roots of cucumber plants (Cucumis sativus) as foraging cues for a specialist herbivore, striped cucumber beetle (Acalymma vittatum) and its natural enemies, entomopathogenic nematodes (EPNs, Heterorhabditis bacteriophora). I predicted HIPVs from A. vittatum‐damaged roots would attract EPNs, while repelling conspecific larvae that avoid competition, and increased risk of predation by EPNs. Finally, I evaluated how olfactory cues emitted by 3 species of EPNs with differing foraging strategies affect the behavior of their insect herbivore prey (A. vittatum) and competing EPNs. I hypothesized olfactory cues from the more sedentary ‘ambush’ EPN species (Steinernema carpocapsae) would be the most repulsive to prey and potential competitors, compared to cues from the active-hunting (H. bacteriophora) or intermediate-foraging (Steinernema riobrave) species. In the second study, I found that 24 hours of wounding by A. vittatum herbivory, or mechanical damage, induced greater production of volatiles from C. sativus roots compared to undamaged controls, repelling foraging larvae and recruiting EPNs. However, after sustained herbivory for 7 days, larvae reduced HIPVs to levels indistinguishable from undamaged roots, while mechanically damaged roots continued to produce higher levels of volatiles. Attenuation of HIPVs impaired C. sativus indirect defenses by reducing recruitment of EPNs and deterrence of A. vittatum larvae. In the final study, I found that foraging A. vittatum larvae avoided olfactory cues from the active-hunting EPN species, Heterorhabditis bacteriophora, but did not respond to cues from the ambush hunter, S. carpocapsae, or intermediate hunter, S. riobrave. In contrast, foraging H. bacteriophora EPNs were attracted to odors produced by the two Steinernema EPN species and did not respond to olfactory cues from conspecifics. Taken together, these results suggest that A. vittatum larvae can navigate risk within the soil environment, first, by avoiding volatile cues associated with increased predation risk or competition ( i.e., volatiles from herbivore-damaged plants or odors from EPN-infected) and, second, through direct attenuation of plant indirect defenses. Our findings also indicate that active-hunting ‘cruiser’ EPNs are attracted to multiple host-associated cues, including volatiles from herbivore-wounded C. sativus roots and odors from heterospefic EPN-infected insect cadavers, suggesting that these cues can provide information for foraging natural enemies about resource availability