REALIZED LIFETIME PARASITISM AND THE INFLUENCE OF BROCHOSOMES ON FIELD PARASITISM RATES OF GLASSY-WINGED SHARPSHOOTER EGG MASSES BY GONATOCERUS ASHMEADI

• Author(s): Hoddle, Mark; Irvin, Nic; Luck, Robert;
• Abstract: GWSS is an exotic pest in California having invaded and established in this state in the late 1980s. One potential reason for the inordinate numbers of GWSS in California compared to population densities in the pests home range in southeastern USA is a lack of an efficient natural enemy fauna that has evolved to use GWSS as a resource. As part of a classical biological control program against GWSS, scientists with the CDFA and UCR have been prospecting for, importing into quarantine, and clearing for release mymarid egg parasitoids from the home range of GWSS for establishment in California. To date, two new parasitoid species have been established in CA, Gonatocerus triguttatus and G. fasciatus. It is too early to ascertain the impact on GWSS population growth that these two parasitoids will have. The self introduced G. ashmeadi (Vickerman et al., 2004) is the key natural enemy of GWSS egg masses in CA at present (Blua et al., 1999). Over summer, parasitism levels of GWSS egg masses and individual eggs in masses by G. ashmeadi approaches 100% but parasitism levels of the spring generation of GWSS are substantially lower (Triapitsyn and Phillips, 2000). Naturally occurring populations of G. ashmeadi in CA have been augmented with mass reared individuals from populations found in the southeastern USA and northeastern Mexico which encompasses the home range of GWSS (D. Morgan - CDFA, pers. comm. 2003). Substantial laboratory work with G. ashmeadi has been conducted in an attempt to understand and parameterize basic aspects of this parasitoids reproductive biology, and host selection behaviors. Irvin and Hoddle (2001) have evaluated oviposition preferences of G. ashmeadi when presented GWSS eggs of various ages. Interspecific competition between G. ashmeadi with G. triguttatus and G. fasciatus for GWSS egg masses of different ages has been assessed along with factors influencing the sex ratio of offspring (Hoddle and Irvin, 2002; 2003). The effect of resource provisioning and nutrient procurement on the longevity of G. ashmeadi has also been determined (Irvin unpublished data). Furthermore, the foraging efficacy of G. ashmeadi in simple and complex environments for scarce and abundant GWSS egg masses has also been completed and compared to similar data collected for G. triguttatus (Irvin unpublished data). The effect of brochosomes on the foraging efficacy of G. ashmeadi has also been evaluated in the laboratory. Brochosomes are a chalky material produced by the malpighian tubules in many xylophagous cicadellid species (Rakitov, 1999; 2000; 2004). Brochosomes are excreted from specialized openings on the posterior of the abdomen and are collected and deposited by mated females on the forewings. During oviposition, females rub brochosomes off the forewings and deposit them on the tops of eggs masses (Hix, 2001). The adaptive significance of covering egg masses with brochosomes is uncertain (Rakitov,1999). Hix (2001) has suggested that brochosomes may protect GWSS eggs from desiccation, UV light, natural enemies (parasitoids, predators and pathogens); or they provide a signal to other female GWSS that leaves have already been oviposited in. We have investigated the effect of brochosomes on the foraging efficacy of G. ashmeadi in the laboratory. Data clearly demonstrate that moderate to heavy brochosome coverage of GWSS eggs is a major impediment to oviposition to G. ashmeadi when compared to conspecific parasitization efficiency of GWSS eggs with light or no brochosome coverage (Velema et al., 2004). Studies currently funded by the CDFA to by conducted by this lab will look at: (1) laboratory-level fecundity rates of G. ashmeadi under varying temperature regimens; (2) field cage studies assessing interspecific competition between parasitoids released for the classical biological control of GWSS; (3) factors affecting sex ratio allocation during mass production of mymarid parasitoids; and (4) the effect of resource provisioning on parasitization rates and overwintering longevity of key mymarid parasitoids under field conditions. The work proposed in this grant will complement and support completed studies and work in progress. Many factors act in concert to affect successful biological control. The GWSS-Gonatocerus system has benefited from intensive laboratory study to generate a basic understanding of factors influencing host selection and parasitism success. The next step that is now required is to test hypotheses generated from lab studies in the field. Field level assessments will evaluate our understanding of the system under investigation, and consolidate interpretations needed to determine the most important aspect of the GWSS biological control program: How effective are egg parasitoids at controlling GWSS in California? To get to the crux of this issue we are asking two questions in this proposal: (1) How big an impact do individual female parasitoids have on GWSS population growth via parasitization of eggs, and (2) do biotic impediments such as brochosomes affect parasitization efficacy in the field? When these two questions are addressed together we will begin to develop a comprehensive understanding of the impacts parasitoids have at the field level and factors affecting parasitization success. This will allow us to form a much better understanding of what levels of control we can expect from mymarid egg parasitoids when different ecological conditions are prevailing in the field.
• Publication Date: Dec 2004
• Journal: 2004 Pierce's Disease Research Symposium