INDOLE AS AN OLFACTORY SYNERGIST FOR VOLATILE KAIROMONES FOR DIABROTICITE BEETLES

Olfactory synergism, where combinations of plant volatile kairomones a re quantitatively more attractive to insects than the sum of attractio n of the individual components, is an important but little-studied phe nomenon in host plant selection and feeding and in pollination ecology . Diabroticite beetles (Coleoptera: Chrysomelidae) are strongly attrac ted to Cucurbita blossoms, and 2- to 5-fold olfactory synergism has be en demonstrated in four species by combinations of the key blossom vol atiles, 1,2,4-trimethoxybenzene, indole, and (E)-cinnamaldehyde, This TIC mixture represents an optimized Curcurbita blossom volatile kairom one mixture useful in monitoring Diabroticite populations and in study ing their behavior and ecology. Indole, which exhibits a spectrum of a ttraction to these beetles ranging from moderate for Diabrotica virgif era virgifera and Acalymma vittatum to very weak for D. barberi, is th e primary synergistic component. Indole combined with 4-methoxycinnama ldehyde was significantly synergistic to D. v. virgifera at a ratio of 1:300 and produced 4-fold synergism at a ratio of 1:1. Indole combine d with 4-methoxyphenethanol was less synergistic to D. barberi with 1. 5- to 2-fold synergism at a 1:1 ratio. These consistent variations in diabroticite beetle olfactory responses presumably indicate evolutiona ry divergences in the numbers of relict indole antennal receptors.