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Ooencyrtus kuvanae, Gypsy Moth Egg Parasitoid

Ooencyrtus kuvanae is a small encyrtid wasp that was first introduced to the eastern United States in 1909 from Japan as an egg parasitoid for control of the gypsy moth. The gypsy moth is a well known defoliator of many hardwood trees, and is itself an exotic insect. In the 1970's and 1980's, many additional introductions of O. kuvanae were made in the northeastern U.S., and after spreading naturally, it is currently found nearly everywhere that the gypsy moth is found.

Females overwinter in the leaf litter, and become active around mid-April. Each oviposits on gypsy moth egg masses for about 4 to 6 weeks, with each female laying an average of 200 eggs. O. kuvanae adults emerge from parasitized eggs between mid-July and early August. This is just about the same time that gypsy moth adult females are emerging, mating, and laying a single egg mass of 100 to 1000 eggs. The new generation of O. kuvanae parasitizes these new egg masses, and the third generation emerges between September and November. In warmer areas, a fourth generation of O. kuvanae is possible in the fall.

O. kuvanae females are good fliers, and they actively search out gypsy moth egg masses on tree trunks and tree branches. The females may locate the egg masses through scents released from the egg masses themselves, or they may be able to detect pheromones of gypsy moth females, and therefore locate egg masses as they are being laid. Field rates of parasitism by O. kuvanae and Anastatus disparis (a less-common egg parasitoid) have been found to vary from region to region. Studies in New Jersey, Virginia, Pennsylvania, and Michigan have shown that, on the average, 20-30% of the gypsy moth egg population is destroyed annually by these egg parasitoids. Monitoring for this egg parasite is fairly easy: parasitized gypsy moth eggs have a tiny round emergence hole.

O. kuvanae has been shown to be susceptible to the insecticides lindane and dimethoate; parasitism is greatly reduced in forests that are treated with these insecticides. The synthetic gypsy moth pheromone Disparlure, which is used for mating disruption, has no effect on parasitism; nor does Bacillus thuringiensis var. kurstaki used to kill larvae.

As an egg parasitoid, O. kuvanae's main benefit is that it has a direct effect on the number of gypsy moth larvae produced, and thus, on the level of defoliation. In addition, O. kuvanae consistently removes a large proportion of potentially reproducing gypsy moths. It is important in maintaining endemic populations, and is an added factor leading to the decline of gypsy moth outbreaks. However, O. kuvanae has a limitation as a biological control agent. Because its ovipositor is quite short, parasitism is restricted to the outermost layers of eggs in a gypsy moth egg mass. So parasitism is inversely related to egg mass size: parasitism rates are higher on small, thin egg masses that are laid on flat surfaces such as smooth-barked trees.

O. kuvanae has become an important factor in gypsy moth egg mortality, and, consequently, in reducing gypsy moth defoliation in northeastern North America. Although it is limited in the number of eggs it can parasitize, O. kuvanae complements the activities of other natural enemies that affect the larval and pupal stages of gypsy moth, such as the fungus Entomophaga maimaiga, the ground beetle Calosoma sycophanta, and many other predators and parasites.

- Amy Christenson, Michigan State University

[Based on information from: Gorvch, C. 1995. Gypsy moth, Lymantria dispar (L.) (Lepidoptera: Lymantriidae), egg mass parasitism by Ooencyrtus kuvanae (Howard) (Hymenoptera: Encyrtidae) in four central Michigan counties. M.S. dissertation, Michigan State University]

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