Olfactory responses of the egg parasitoid, Gonatocerus ashmeadi Girault (Hymenoptera: Mymaridae), to host plants infested by Homalodisca vitripennis (Germar) (Hemiptera: Cicadellidae) Rodrigo Krugner a, * ,1 , Marshall W. Johnson a , Kent M. Daane b , Joseph G. Morse a a Department of Entomology, University of California, Riverside, CA 92521, USA b Department of Environmental Science, Policy and Management, University of California, Berkeley, CA 94720, USA article info Article history: Received 15 November 2007 Accepted 10 June 2008 Available online 17 June 2008 Keywords: Egg parasitoids Tritrophic interactions Olfactory response Oviposition Host location Biological control abstract Gonatocerus ashmeadi Girault is an egg parasitoid with potential use in glassy-winged sharpshooter (GWSS), Homalodisca vitripennis (Germar), biological control 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 ovipo- sition. Using a glass Y-tube olfactometer, we determined G. ashmeadi’s preference for volatiles of infested versus uninfested lemon, 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. G. ashmeadi chose the infested lemon, red-tip photinia, or grapevine plant significantly more often (62%, 67%, and 63%, respectively) than uninfested hosts, whereas they chose infested and uninfested crape myr- tle equally often (49%). Females spent more time in and made more visits to the olfactometer arm con- nected to the infested plant than to the arm connected to the uninfested plant, except when testing crape myrtle. When testing for a systemic plant response to GWSS infestation, only infested lemon was signif- icantly more attractive to the parasitoids than was an uninfested plant. These results suggest that G. ash- meadi females use chemical volatiles as cues to find GWSS egg masses and that host finding success varies among GWSS host plants. Some plants used by GWSS as oviposition hosts may serve as an escape from parasitization by G. ashmeadi and act as reservoirs for new GWSS individuals. Published by Elsevier Inc. 1. Introduction The glassy-winged sharpshooter (GWSS), Homalodisca vitripennis (Germar), is a cicadellid native to the southeastern United States and northeastern Mexico (Triapitsyn and Phillips, 2000) and was first de- tected in California in 1990 (Sorensen and Gill, 1996). It is a highly polyphagous leafhopper with over 100 known hosts (Turner and Pol- lard, 1959; Sorensen and Gill, 1996). It probably entered California from the southern or southeastern USA via egg masses imported on nursery stock (Sorensen and Gill, 1996). After its establishment, GWSS vectored and rapidly spread the endemic bacterium, X. fastidi- osa Wells et al., throughout vineyards in the Temecula Valley of southern California (Blua et al., 1999) and the southern San Joaquin Valley (Hashim and Hill, 2003). Xylella fastidiosa is the causal agent of Pierce’s disease (PD) in grapes (Davis et al., 1978), economically one of the most important crops in California ($4.14 billion/year) (CDFA, 2006). Currently, PD infections are managed by removing diseased grapevines (Hashim and Hill, 2003) and by reducing GWSS populations in citrus groves, urban areas, and vineyards using insec- ticides and biological control agents (Wendel et al., 2002; Hix et al., 2003). Biological control agents which have been mass produced and released in California to suppress GWSS populations include the mymarid egg parasitoids: Gonatocerus ashmeadi Girault, G. fasciatus Girault, G. morrilli (Howard), G. triguttatus Girault, G. walkerjonesi Triapitsyn, and Anagrus epos Girault (CDFA, 2003; Morse, 2006). G. ashmeadi was found to be the most common egg parasitoid of GWSS in California, Florida, and Louisiana (Triapitsyn et al., 1998) and currently exhibits a high potential for suppressing GWSS populations. It was first detected in California in 1978, and was probably unintentionally reintroduced one or more times during the early phases of GWSS’ invasion as suggested by the genetic similarities and reproductive compatibilities among individuals collected throughout the USA and Mexico (Vickerman et al., 2004). Since the invasion of GWSS in California, various aspects of the biology and ecology of G. ashmeadi have been studied extensively including its ovipositional behavior (Velema et al., 2005), reproductive and developmental biology at various temperatures (Chen et al., 2006b; Pilkington and Hoddle, 2006), its functional response and superparasitism (Chen et al., 2006a), its overwinter- 1049-9644/$ - see front matter Published by Elsevier Inc. doi:10.1016/j.biocontrol.2008.06.004 * Corresponding author. Fax: +1 559 596 2921. E-mail address: Rodrigo.Krugner@ars.usda.gov (R. Krugner). 1 Present address: USDA-ARS, San Joaquin Valley Agricultural Sciences Center, Parlier, CA 93648, USA. Biological Control 47 (2008) 8–15 Contents lists available at ScienceDirect Biological Control journal homepage: www.elsevier.com/locate/ybcon