Differential host choices of Helicoverpa armigera and H. assulta to tomato

The specialist Helicoverpa assulta and the generalist Helicoverpa armigera are closely related species in Noctuidae. To elucidate the chemical basis underlying their differential oviposition responses to tomato, we compared the oviposition and walking responses of adult females and larvae of both species to tomato volatiles as well as derived synthetic blends. Dichloromethane and hexane extracts of tomato leaves deterred oviposition by H. assulta females. In contrast, all three extracts significantly attracted oviposition by H. armigera females. The components of the dichloromethane extract of tomato leaves were classified according to biosynthetic pathways (acetylmalonate, acetyl‑mevalonate, and shikimate pathways), and four blends were prepared based on their relative percentages in the extract: Blend 1 (green leaf volatiles), Blend 2 (aromatic compounds), Blend 3 (terpenoids), and Blend 4 (the full blend, contained all components that contained in the Blends 1 ~ 3). All blends significantly deterred oviposition by H. assulta females, with Blend 4 being the most effective. Only Blends 2 and 3 significantly attracted oviposition by H. armigera females. Larval walking bioassays on a locomotion compensator indicat that the differential host use is attributable to adult oviposition preference, rather than to larval olfactory discrimination between host and non-host volatiles. In field trials, the full blend effectively reduced egg deposition by H. assulta rather than H. armigera on tobacco plants. All together, the oviposition deterrence of H. assulta by tomato appears to be regulated by a “partial blend” of compounds commonly found across the plant kingdom, not by tomato-specific unique components.