Host-associated kairomones used for habitat orientation in the parasitoid Lariophagus distinguendus (Hymenoptera: Pteromalidae)

Abstract

Males and females of the parasitic wasp Lariophagus distinguendus respond to volatiles emitted by the larval faeces of one of their hosts, the granary weevil Sitophilus granarius. Previous studies have shown that attractive chemicals are emitted by astigmatid mites living in the host faeces and that these cues are attractive only to experienced parasitoids. In the present study we demonstrate that larval faeces of the host and headspace extracts of the faeces are attractive for both sexes of the parasitoid even when the mites were experimentally excluded from the beetle rearings. The response to volatiles from mite-free host faeces is innate. In order to elucidate the chemistry of this odor, headspace extracts were fractionated by adsorption chromatography. Tests using combinations of fractions of different polarities revealed that both the non-polar pentane and the polar methanol fractions were necessary to maintain the attractiveness. This indicates that the attractive odor is composed of a complex blend of components with different polarities. The composition of the polar fraction was analyzed by gas chromatography-mass spectrometry, whereas elucidation of the structure of non-polar components has not been possible so far. By orientating toward the same host-related volatiles used by females for host finding, L. distinguendus males may be arrested in patches of potentially high female density and thus increase their chance of mating.