Feeding by a gall-inducing caterpillar species alters levels of indole-3-acetic and abscisic acid in Solidago altissima (Asteraceae) stems

Various plant antagonists appear to alter phytohormone levels for their own benefit. Among insects, gall-inducing species appear to influence phytohormones and it is widely believed that they alter levels of indole-3-acetic acid (IAA) to help produce their galls, but evidence exists for only a limited number of species. To further explore the role of phytohormones in gall formation, we measured levels of IAA and abscisic acid (ABA), a hormone involved in plant defenses and that can influence IAA, in tissues of control stems of Solidago altissima (Asteraceae) and those galled by Gnorimoschema gallaesolidaginis (Gelechiidae). This gall-inducing caterpillar species significantly altered the distribution of IAA in galls and the larvae themselves contained high concentrations of IAA. In contrast, the generalist caterpillar Heliothis virescens (Noctuidae) neither altered IAA nor accumulated significant concentrations of IAA, suggesting that G. gallaesolidaginis may have a distinctive influence over IAA. The gall-inducing caterpillars, particularly younger larvae, also contained high levels of ABA but did not increase levels of ABA, which is induced by herbivory of H. virescens. Because G. gallaesolidaginis also does not increase levels of other defense-related hormones, avoiding generalized plant defenses may facilitate gall induction and formation.