Know Your Friends

Cotesia melanoscela, A Parasitoid of Gypsy Moth

This braconid wasp was one of the first parasitoids introduced from Europe early in this century to control the gypsy moth. There are actually two strains of the wasp in the U.S. now. The European strain was brought from Italy in 1912, while the Asian strain was introduced from India in 1977 and from South Korea in 1983. This species is now widespread and abundant where gypsy moth is established, and is one of the principal parasitoids attacking early-instar gyspy moths in North America.

The shiny black adults are 3mm long. There are two generations per year, each taking about 2-3 weeks. C. melanoscela overwinters as full-grown larvae in their sulfur-yellow cocoons. Adults emerge when gypsy moth caterpillars start hatching from their overwintering eggs. Each female lays 500 to 1,000 eggs, which she places singly in first and second instar gypsy moth larvae. Oviposition itself may be enough to kill small caterpillars. During the second generation, the females oviposit in third instar caterpillars as well, but later instars are not easily parasitized because of their long hairs and vigorous defensive movements. When the larvae complete their development, they tear a hole in the side of the caterpillar and emerge from their hosts. They spin silken cocoons on the foliage, tree bark, or any other objects the caterpillars crawled onto before dying.

The overwintering generation of C. melanoscela is attacked by many hyperparasites, which reduce the effectiveness of this wasp. These other parasitoids can parasitize up to 80-90% of the overwintering C. melanoscela cocoons. Although this high mortality does not prevent the wasp from establishing and sustaining a population, it does reduce the ability of C. melanoscela to suppress gypsy moth populations.

Experimental inundative releases have given mixed results, probably because of differences in the size of the gypsy moth infestation, numbers of wasps released, the amount of parasitism from native C. melanoscela, and methods of evaluation. Improved efficacy may come through better distribution and application techniques.

In many areas gypsy moth populations are suppressed during federal and state cooperative programs by aerial applications of various insecticides. When gypsy moth larvae are sprayed with the microbial insecticide Bacillus thuringiensis (Bt) they take longer to develop. This allows C. melanoscela to attack more caterpillars, mainly because they are in the small, susceptible instars longer. Bt and C. melanoscela act synergistically and provide greater foliage protection than either treatment alone.

Gypcheck®, a formulation of gypsy moth nuclear polyhedrosis virus (NPV), is also applied aerially. It is specific for gypsy moth (unlike Bt which can kill many nontarget caterpillars) and persists for a long period of time so subsequent generations of gypsy moth can be infected. However, it is not as compatible with C. melanoscela as Bt is because the wasps avoid NPV-infected gypsy moth larvae. Also, wasp larvae developing inside caterpillars that become infected probably cannot complete their development before the host dies from the virus infection, so C. melanoscela populations are reduced in areas treated with Gypcheck.

The insect growth regulator diflubenzuron (Dimilin®) is another insecticide commonly used in aerial application programs. Parasitized gyspy moth larvae treated with Dimilin ususally die before the parasitoid completes its development, decreasing C. melanoscela populations.

- Susan Mahr, University of Wisconsin-Madison

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