Competition between herbivore feeding guilds on cotton plants is species specific and likely plant-mediated

Interspecific competition among herbivorous insects is often mediated by a common host plant. Changes in the common host plant induced by one herbivore species may make the plant less preferred or nutritious to another herbivore. We suggest that these interactions can be quite specific. We examined the pair-wise interactions between species of two different feeding guilds on the same host plant. Cotton, Gossypium hirsutum, responds to herbivory by the Heliothine caterpillars Helicoverpa zea and Heliothis virescens [Lepidoptera: Noctuidae] through the induction of tissue-bound and volatile systemic compounds, specific to the Heliothine species. The stink bug species Nezara viridula and Euschistus servus [Hemiptera: Pentatomidae] are also common herbivores of cotton. We found that the strength of competition between Heliothine and stink bug species depended on the species of Heliothine, the species of stink bug, and the spatial scale on which the interaction took place. Our results suggest that plant induction of anti-herbivore compounds, as a result of Heliothine feeding, deters stink bug feeding and oviposition. We are presently examining the chemical basis of these effects in stink bug choice tests to understand the mechanisms of competition. In systems where inter-guild competition is species specific, the mechanisms driving competition should not be properties of the guilds in question. Therefore, determining differences in the responses by herbivores at the species level to induced plant changes may be a useful way to gain an understanding of the mechanism(s) of competition.