Indole as an olfactory synergist for volatile kairomones for diabroticite beetles

Olfactory synergism, where combinations of plant volatile kairomones are quantitatively more attractive to insects than the sum of attraction of the individual components, is an important but little-studied phenomenon in host plant selection and feeding and in pollination ecology. Diabroticite beetles (Coleoptera: Chrysomelidae) are strongly attracted toCucurbita blossoms, and 2- to 3-fold olfactory synergism has been demonstrated in four species by combinations of the key blossom volatiles, 1,2,4-trimethoxybenzene, indole, and (E)-cinnamaldehyde. This TIC mixture represents an optimizedCurcurbita blossom volatile kairomone mixture useful in monitoring Diabroticite populations and in studying their behavior and ecology. Indole, which exhibits a spectrum of attraction to these beetles ranging from moderate forDiabrotica virgifera virgifera andAcalymma vittatum to very weak forD. barberi, is the primary synergistic component. Indole combined with 4-methoxycinnamaldehyde was significantly synergistic toD. v. virgifera at a ratio of 1:300 and produced 4-fold synergism at a ratio of 1:1. Indole combined with 4-methoxyphenethanol was less synergistic toD. barberi with 1.5- to 2-fold synergism at a 1:1 ratio. These consistent variations in diabroticite beetle olfactory responses presumably indicate evolutionary divergences in the numbers of relict indole antennal receptors.