Submitted to: Biological Control
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/4/2014
Publication Date: 7/2/2014
Citation: Krugner, R., Wallis, C.M., Walse, S.S. 2014. Attraction of the egg parasitoid, Gonatocerus ashmeadi Girault (Hymenoptera: Mymaridae) to synthetic formulation of a (E)-ß-ocimene and (E,E)-a-farnesene mixture. Biological Control. 77:23-28. Interpretive Summary: Pierce’s disease is a serious disease of grapevine caused by the bacterium Xylella fastidiosa, which can be vectored by the glassy-winged sharpshooter (GWSS). To reduce incidence of Pierce’s disease, efforts are underway to suppress GWSS populations through the use of pesticides as well as mass releases of egg parasitoids. However, little is known about the chemical cues that parasitoids use to find GWSS eggs, even though such knowledge could be useful to improve effectiveness of parasitoid mass releases. Grapevines could respond to GWSS feeding or oviposition by changing volatile compound levels, which parasitoids could cue in on to find GWSS eggs. This study evaluated differences in volatile compound levels between non-infested and infested grapevines, with infested grapevines either only fed upon by GWSS or fed upon with GWSS oviposition occurring as well. GWSS feeding by itself did not affect volatile compound production. However, grapevines infested with GWSS egg masses produced greater levels of three volatile compounds: 1,8-cineole, ß-ocimene, and a-farnesene. In Y-tube olfactometer assays, the blend containing (E)-ß-ocimene and (E, E)-a-farnesene attracted the female egg parasitoid Gonatocerus ashmeadi Girault, whereas these two compounds tested alone, in paired combinations with 1,8-cineole, and mixed into a three-compound blend did not result in a significant increase in female response behavior. Previous studies found that non-GWSS parasitoids can use these compounds to find plants infested with their hosts, and it is likely that these volatiles could also attract GWSS egg parasitoids. If this is the case, these compounds could be used as lures to attract GWSS parasitoids to heavily-infested sites or be used in traps to monitor local parasitoid levels.
Technical Abstract: Glassy-winged sharpshooter (GWSS), Homalodisca vitripennis (Germar), is a vector of the xylem-inhabitant bacterium Xylella fastidiosa Wells et al., which causes Pierce’s disease of grapevines. Current GWSS control strategies are limited to area-wide insecticide applications and mass release of egg parasitoids. Key to the effectiveness of parasitism-based control is mechanistic knowledge of chemical mediation of this multitrophic interaction, including the characterization of chemical volatiles that attract parasitoids to GWSS eggs. Volatiles emitted from non-infested or GWSS-infested grapevines contained in a glass chamber were immobilized on a sorbent cartridge and analyzed with gas chromatography. Volatile profiles from non-infested grapevines did not differ from grapevines on which GWSS only fed. However, three volatile terpenoids, identified as 1,8-cineole, (E)-ß-ocimene, and (E, E)-a-farnesene, were emitted at approximately two-fold greater levels from grapevines with GWSS egg masses than the non-infested plants. In Y-tube olfactometer assays, the blend containing (E)-ß-ocimene and (E, E)-a-farnesene attracted the female mymarid egg parasitoid Gonatocerus ashmeadi Girault, whereas these two compounds tested alone, in paired combinations with 1,8-cineole, and mixed into a three-compound blend did not result in a significant increase in female response behavior.