Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/10/2008
Publication Date: 6/17/2008
Citation: Krugner, R., Johnson, M.W., Daane, K.M., Morse, J.G. 2008. Olfactory responses of the egg parasitoid, Gonatocerus ashmeadi Girault (Hymenoptera: Mymaridae), to host plants infested by Homalodisca vitripennis (Germar) (Hemiptera: Cicadellidae). Biological Control. 47:8-15. Interpretive Summary: Gonatocerus ashmeadi Girault is the most common egg parasitoid of glassy-winged sharpshooter (GWSS) in California and currently exhibits a high potential for suppressing GWSS populations. Various aspects of the biology and ecology of G. ashmeadi have been studied extensively, and have provided important information on the interaction of G. ashmeadi with GWSS. However, little is known about the parasitoid’s host location process, which might be the most difficult and time-consuming phase for a female parasitoid. This process can be facilitated by the use of chemical volatiles (i.e., “odors”) that function as signals indicating the presence of hosts. We tested the hypotheses that 1) G. ashmeadi uses chemical volatiles as cues to find GWSS egg masses, 2) its success in host finding varies among the numerous GWSS host plants, and 3) the release of chemicals is systemically induced by GWSS feeding and oviposition. We determined G. ashmeadi’s preference for volatiles of infested versus uninfested citrus, grapevine, red tip photinia, and crape myrtle plants. The proportion of responding females was significantly different among the host plant species. When exposed to plants exhibiting GWSS feeding and egg masses versus an uninfested plant, females initially chose the infested citrus, red tip photinia, and grapevine plant. In addition, females spent more time in and made more visits to the infested versus the uninfested plant. Infested and uninfested crape myrtle plants were equally attractive to the female wasps. These results suggest that G. ashmeadi uses chemical volatiles as cues to find GWSS egg masses and the host finding success of G. ashmeadi females varies among GWSS host plants. The use of synthetic versions of chemicals volatiles and use of crop varieties that are more attractive to parasitoids are discussed in terms of potential control strategies for GWSS.
Technical Abstract: Gonatocerus ashmeadi Girault is an egg parasitoid that exhibits potential for managing glassy-winged sharpshooter (GWSS), Homalodisca vitripennis (Germar), populations in California. We tested the hypotheses that 1) G. ashmeadi uses chemical volatiles as cues to find hosts, 2) its success in host finding varies among plant species, and 3) the release of semiochemicals is systemically induced by GWSS feeding and oviposition. Using a glass Y-tube olfactometer, we determined G. ashmeadi’s preference for volatiles of infested versus uninfested citrus, grapevine, red tip photinia, and crape myrtle plants. The parasitoid’s first choice to, its residence time, and the number of visits per Y-tube arm were used to assess its plant choice. Gonatocerus ashmeadi chose the infested citrus, red tip photinia, or grapevine plant significantly more often (62%, 67%, and 63% respectively), whereas they chose infested and uninfested crape myrtle equally often (49%). Females spent more time in and made more visits to the olfactometer arm connected to the infested plant than to the arm connected to the uninfested plant. When testing the effect of GWSS feeding alone, G. ashmeadi manifested a 1:1 ratio in the proportion of first choice, visits, and residency times. When testing for a systemic plant response to GWSS infestation, only infested citrus was significantly more attractive to the wasps than was an uninfested plant. These results suggest that G. ashmeadi females use chemical volatiles as cues to find GWSS egg masses and that host finding success varies among GWSS host plants.