Indole sensitivity in various insect herbivores.

When plants in nature are attacked by herbivores, they frequently release airborne volatiles. These herbivore-induced plant volatiles (HIPVs) help plants defend themselves against herbivore attack by calling in natural enemies, but HIPVs may also be directly toxic to herbivores. Indole (C8H7N) is a common HIPV that is toxic to caterpillars, but its relative toxicity to different caterpillars has not been explored. I tested the toxicity of indole on six caterpillar species (Spodoptera frugiperda, Trichoplusia ni, Spodoptera exigua, Anticarsia gemmatalis, Heliothis virescens, and Helicoverpa zea). Results show that indole is most toxic to Trichoplusia ni and Anticarsia gemmatalis, moderately toxic to Heliothis virescens, and least toxic to Spodoptera frugiperda, Spodoptera exigua, and Helicoverpa zea. The results of this study suggest that there is a relationship between indole sensitivity and host range of a caterpillar species, and that species with narrower host ranges are more sensitive to indole. This information provides a useful foundation for studying plant defenses, because the toxicity of most plant defense compounds is unknown for most herbivore species