We describe a new and hitherto unreported function for changes in plant chemistry induced by phytophagous insects: to provide cues for mate location. Larvae of the gall wasp Antistrophus rufus Gillette (Hymenoptera: Cynipidae) feed within inconspicuous galls inside the flowering stems of the prairie perennials Silphium laciniatum L. and S. terebinthinaceum Jacquin (Asteraceae). Adult male A. rufus emerge before females and are challenged with locating mates that are sequestered within dead plant stems that occur in a matrix of dead vegetation.
Allozyme studies revealed complete reproductive isolation between wasp subpopulations in the two plant species. In laboratory bioassays, males responded only to their natal plant species, antennating the stem surface. Males from S. laciniatum also preferred galled stems of S. laciniatum to ungalled stems and responded to hexane extracts of galled stems. These extracts contained much higher concentrations of monoterpenes than did S. terebinthinaceum. Ratios of “+” and “-” enantiomers of alpha-pinene and beta-pinene approximated 50:50 for ungalled S. laciniatum stems but strongly differed from 50:50 in galled stems, with “+” and “-” enantiomers strongly dominant in different plants. In bioassays, male wasps from S. laciniatum responded to a synthetic blend of the monoterpenes in enantiomeric ratios characteristic of galled stems.
Male A. rufus rely entirely on olfaction to locate females within stems in a complex prairie habitat, and gall wasps themselves apparently influence the plant to modify ratios of monoterpene enantiomers. These plant volatiles serve as a signal for males, serving as a sex pheromone proxy for females concealed within plant tissues.
Species 1: Hymenoptera Cynipidae Antistrophus rufus
Keywords: gall, mate location
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