OF THE GLASSY-WINGED SHARPSHOOTER : FUNCTIONAL RESPONSES AND SUPER-PARASITISM BY THE EGG PARASITOID GONATOCERUS ASHMEADI

The functional responses and super-parasitism by the egg parasitoid, Gonatocerus ashmeadi, on Homalodisca coagulata eggs were related to host age and density when studied under laboratory conditions. Parasitism of Glassy-winged Sharpshooter (GWSS) eggs, 1-, 3-, 5-, 7and 9-d-old, was measured at 22 ± 1oC and under 10L:14D regime. For each host age, 10-60 eggs were exposed to an individual parasitoid for 24 h. The functional responses for the parasitoids to host eggs of all age groups most closely fit the type II and III models of Hollings (1959) and Hassell (1978) which relate to the elapsed time for accomplishing the behavioral events associated with parasitism of the host as modified by host density. The instantaneous attack rate by parasitoids on 1-d-old host eggs, as specified in the type III model, was significantly greater from that of the other ages. This rate was also greater in the type II model but was not statistically significant. The total number of host eggs parasitized varied significantly with host density and age of the eggs, but not when analyzed by a host x density interaction. Host age and density, as well as the host x density interaction, contributed significantly to the differences found in length of development time of G. ashmeadi within host eggs. The wasps exhibited a tendency towards super-parasitism at relatively high parasitoid-to-host ratios. The maximum number of parasitoid eggs found in a single host egg was 18. The development time and eclosion of the parasitoids had no correlation with parasitoid-to-host ratios. Frequencies of super-parasitism for G. ashmeadi displayed an aggregated distribution over all observed host densities. INTRODUCTION The effectiveness of parasitoids in regulation of a pest population is highly dependent on their ability to search for and handle hosts in a varying ecosystem. This effectiveness has been traditionally related to the functional response of a parasite or predator (Hassell 1978, Fujii et al. 1986). The functional response is defined as the relationship between the numbers of prey taken by the predator as a function of prey density (Holling 1959). The functional response is an essential component of the dynamics of host-parasitoid relationship, and is an important determinant of the stability of the system (Oaten and Murdoch 1975). Functional response analyses are commonly used to help predict the potential for parasitoids to regulate host population (Solomon 1949, Oaten and Murdoch 1975). Successful parasitoids have the ability to discriminate among parasitized eggs, avoid super-parasitism and minimize the waste of time and energy associated with their searching and parasitizing behaviors (Godfray 1994). However, under certain circumstances, superparasitism might be adaptive (van Alphen & Visser 1990). Further, when mass-rearing solitary parasitoids for use in an augmentative release program, superparasitism represents a waste of the production colony’s potential output. This report presents the progress on investigations determining certain aspects of the functional responses and super-parasitism by the parasitoid, G. ashmeadi. OBJECTIVES 1. Investigate the response of G. ashmeadi to GWSS eggs of different ages and determine the effects of host egg age on functional response parameters and parasitism. 2. Determine effect of host densities and ages with respect to developmental time of wasps. 3. Investigate relationship between super-parasitism by the wasp at different host densities and effect of super-parasitism on wasp emergence and development time. RESULTS Functional Responses There was a significant increase in the numbers of H. coagulata eggs of different ages parasitized by egg parasitoid, G. ashmeadi, with an increase in host density (Table 1). At the host densities of 40, 50, and 60, the numbers of eggs parasitized were significantly higher than that of relatively low densities of 10 and 20 over all host ages. The number of 1-d-old eggs parasitized was slightly greater than that of 5-, 7and 9-d–old-eggs. A two-way ANOVA, with age and density as factors, revealed that the number of eggs parasitized varied significantly with host age (F = 3.64, df = 4,299, P = 0.007) as well as host densities (F = 88.43, df = 5,299, P < 0.0001). There was no significant effect of age × density interaction on the number of host eggs parasitized (F = 0.44, df = 20, 299, P = 0.899). The functional responses of G. ashmeadi parasitizing host eggs at the various ages showed that the shape of the functional response curves were affected by differences in the parasitization rates of G. ashmeadi. At all host ages, the G. ashmeadi functional response data most closely fit the type II and III models. Coefficients of determination (r values) for type II and III curves were very similar (Table 2). The instantaneous attack rates (a) and handling time (Th) estimated