Plant surfaces of vegetable crops mediate interactions between chemical footprints of true bugs and their egg parasitoids

Abstract

During the host location process, egg parasitoids can eavesdrop on chemical cues released from immature and adult hosts. These indirect host-related cues are highly detectable, but of low reliability because they lead egg parasitoid females to an area where oviposition is likely to occur rather then providing wasps with direct information on the presence of eggs and their location. In the host-parasitoid associations between true bugs and their scelionid egg parasitoids, female wasps perceive the chemical residues left by host adults walking on substrates as contact kairomones, displaying a characteristic arrestment posture. In this study, we demonstrated that epicuticular waxes of leaves of two vegetable crops, broad bean, Vicia faba, and collard greens, Brassica oleracea, mediate the foraging behavior of Trissolcus basalis (Wollaston) by adsorbing contact kairomones from adults of Nezara viridula (L.). Trissolcus basalis females showed no response when released on the adaxial leaf surface of broad bean or collard green plants with intact cuticular wax layers that had not been exposed to bugs, whereas wasps displayed the arrestment posture when intact leaves were contaminated by chemical residues from host females. Adaxial leaf surfaces that were dewaxed with an aqueous solution of gum arabic and afterwards contaminated by N. viridula females elicited no arrestment responses from wasp females. Similarly, leaves contaminated by host females and subsequently dewaxed did not elicit responses from female wasps. These findings reveal the important role of plant waxes in N. viridula - T. basalis semiochemical communication.

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Acknowledgements

Funding for this work was provided by “Progetti di ricerca di interesse nazionale—cofinanziamento 2007” entitled “Enhancing foraging behaviour of insect egg parasitoids: the role of volatile organic compounds and the epicuticular layers of plants” Chairperson Prof. Stefano Colazza. Gianandrea Salerno provided the drawing. We thank Jocelyn Millar for critically reading and commenting the manuscript.

Figures and Tables

Figure 1 Overview of the chemical cues involved in the host location process in the host-egg parasitoid associations of true bugs and scelinoid wasps. To find host eggs, parasitoids can use semiochemicals originating from the host eggs (direct host-related cues) or indirect host-related cues from adults of the host species.

Figure 2 Effects of adaxial leaf epicuticular waxes of Vicia faba and Brassica oleracea on Trissolcus basalis response to Nezara viridula female footprints that constitute a contact kairomone. Green bars indicate the percentage of wasps that displayed arrestment responses after being released; red bars represent the percentage of those that did no show arrestment responses. Leaves were contaminated by allowing 2–3 mated, gravid N. viridula to walk over them for 30 min. Leaves were mechanically dewaxed with an aqueous solution of gum arabic. Asterisks indicate significant differences at p < 0.01 (Pearson χ² test).

Figure 3 Scanning electron micrographs of the adaxial leaf surface of Vicia faba (A) and Brassica oleracea (B) with the epicuticular waxes partly removed after a single treatment with gum Arabic, revealing the smooth surface of the cutin matrix. The line between the dewaxed area (bottom half of the pictures) from the neighbouring intact area is highlighted by arrows.

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