Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/29/2005
Publication Date: 4/1/2006
Citation: Chen, W., Leopold, R.A., Harris, M. 2006. Parasitism of the glassy-winged sharpshooter, Homalodisca coagulata (Homoptera: Cicadellidae): Functional reponse and superparasitism by Gonatocerus ashmeadi (Hymenoptera: Mymaridae). Biological Control. 37(1):119-129.
Interpretive Summary: This study reports on the effectiveness of an egg parasite, a wasp, when exposed to eggs of the glassy-winged sharpshooter, a leaf hopper that is a vector of Pierce's Disease in a number of agriculturally important crops. Several aspects of parasitism were measured, such as the amount of time that the wasp, Gonatocerus ashmeadi, spends parasitizing host eggs, the number of eggs parasitized when given different densities of eggs, and the number of parasite eggs deposited per host egg. Collectively, these different parameters of parasitism are termed functional responses. There are several types of functional responses and the interaction of this wasp with the eggs of the sharpshooter is best fit to type II and type III functional response models. The study also showed that the number of host eggs parasitized by the wasp varies significantly with the density and age of the eggs. The attack rate by the wasp is greater on 1 day-old eggs than older eggs and the number of progeny placed into each host egg by the wasp is greater at low ratios (1:10) than at high ratios (1:60). The information provided by this study can be used for establishing quality control standards for mass-reared insects and also in an augmentative field release of the wasp in a biological control program.
Technical Abstract: The functional response by the egg parasitoid, Gonatocerus ashmeadi, and superparasitism of Homalodisca coagulata eggs were found to be related to host age and density when studied under laboratory conditions. Several aspects relating to parasitism of 1-, 3-, 5-, 7-, 9-d-old Homalodisca coagulata eggs were measured under varied densities ranging from 1:1 to 1:60 parasitoid to host ratios. The functional responses for the parasitoid to host eggs of all age groups most closely fit the type II and III models that describe responses to changing densities. The instantaneous attack rate by the parasitoids on 1-d-old hosts, as specified in the type III model, is significantly greater than that of other ages. This rate is also greater in the type II model but is not statistically significant. The number of host eggs parasitized varies significantly with host density and age, but not when analyzed by a host age × density interaction. However, host age and density, as well as the host age × density interaction, contribute significantly to the differences found in length of the development time of G. ashmeadi within host eggs. This parasitoid showed a significantly greater tendency toward superparasitism at parasitoid-to-host ratios'1:10. The maximum number of parasitoid eggs found in a single host egg was 18. The frequencies of superparasitism for G. ashmeadi display an aggregated distribution over all observed host densities. Our results also suggest G. ashmeadi eliminates the supernumerary parasitoids through physiological suppression.