Abstract
The behavioral response of the larval parasitoid Spintherus dubius (Hymenoptera: Pteromalidae) to volatile compounds derived from its Apion weevil hosts was investigated in two-choice bioassays. Odor source candidates were the larval and adult stages of weevils, clover flowers, and feces from adult weevils. Despite S. dubius being a larval parasitoid, the odor of weevil larvae isolated from the clover flowers was not attractive to female parasitoids. Surprisingly, S. dubius females were instead attracted by the odor from the feces of adult weevils. The female parasitoids were similarly attracted to the feces produced by the two main hosts, the red clover weevil (A. trifolii) and the white clover weevil (A. fulvipes). Chemical analysis of the volatile composition of feces produced by the two hosts revealed qualitatively similar odor profiles, correlating with the observed attraction by the parasitoid towards both odor sources. Some of the identified volatile compounds are commonly present in clover plant headspace fractions and may function as a kairomone to facilitate orientation by S. dubius to Apion-infested clover flowers. Larval and adult weevils were not attractive for parasitoid females, whereas, for the white clover weevil-plant association, infested flowers were highly attractive. These data show the use by the clover weevil parasitoid of an alternative source of olfactory information for locating its host.
Highlights
Insect herbivores as well as their natural enemies rely heavily on olfactory cues for locating food and oviposition sites, even at great distances
In experiments associated with the A. trifolii-T. pratense complex, significant attraction of female parasitoids towards the feces was confirmed when they were tested against weevil larvae (N = 27; χ2 = 4.5; df = 1; P < 0.05), or against non-infested clover flowers (N = 28; χ2 = 5.1; df = 1; P < 0.05)
In experiments associated with the A. fulvipes-T. repens complex, a significant attraction towards the weevil feces was recorded when that stimulus was paired with the odor from infested clover flowers (N = 32; χ2 = 8; df = 1; P < 0.01)
Summary
Insect herbivores as well as their natural enemies rely heavily on olfactory cues for locating food and oviposition sites, even at great distances. These weevils overwinter below dead leaves or grass, and in the spring overwintered adults emerge and locate host plants where they feed and mate (Freeman 1967). Mature weevils start to mate during the early flowering phenology of the clover host, and females oviposit in the developing clover head. The weevil larvae feed on the clover seeds and they stay inside the floret/flower head during all their development (Notini 1935; Kruess and Tscharntke 1994; Nyabuga et al 2015). They pupate inside the flower, and only when they reach their adult stage, they leave the flowers and emerge as young weevils
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