Know Your Friends

Compsilura concinnata, Parasitoid of Gypsy Moth

The tachinid fly Compsilura concinnata is a parasitoid of gypsy moth and other imported pests, such as satin moth and brown-tail moth. It was one of more than 45 species of natural enemies introduced for control of gypsy moth over a period of over 50 years, beginning in 1906. This native European species is highly polyphagous and has been reared from more than 200 host species in the United States. Some are pests such as forest tent caterpillar, but many are non-pest species in the families Nymphalidae (brushfooted butterflies) or Saturniidae (giant silkworms).

The blackish adult fly is 7.5 mm long with four deep black stripes on the thorax and lots of bristles. In the spring the adults emerge to larviposit in caterpillars. The female approaches a host, moving closely around it for a few seconds. She then darts at its middle, pinches the host's body between spines on the underside of her abdomen and her sharp larvipositor, and injects a young maggot into the wound-in a single motion that takes about second. She often examines and attacks the same host a number of times. Each female produces about 100 larvae. The maggot develops within the intestine or body cavity of the caterpillar, and after consuming the contents, leaves the host to pupate in the soil, in the host web, or in crevices in the bark of the tree the host was feeding on. Adult flies emerge in about 10 days. During the summer the cycle from larviposition to adult emergence can be completed in about 20 days. There are up to four generations per year, with one generation developing when gypsy moths are vulnerable to parasitism. The fly overwinters in the immature stage in living caterpillars of various species (but not in gypsy moth since it overwinters in the egg stage).

The abundance of C. concinnata in the spring, and consequently the degree of parasitism of gypsy moth, is influenced by the abundance of its hibernating hosts. Most of these host insects pass the winter in the pupal stage, either above or below ground. Overwintering hosts include Diacrisia virginica, Apatela americana, Ampelophaga myron, Mamestra picta, Sphinx gordius, promethea moth, black swallowtail, spicebush swallowtail and others. In the summer numerous species in widely divergent habitats in 18 families of Lepidoptera are utilized as hosts.

Parasitization of gypsy moth by C. concinnata is generally low (less than 5%) during gypsy moth outbreaks, but tends to be higher when pest populations decline. Parasites contribute significantly to mortality of gypsy moth larvae, but C. concinnata is often less effective than other species of parasitoids. Its effects are diluted because it attacks so many other hosts, and it also prefers to attack hosts on low foliage, so many of the caterpillars higher in the trees escape parasitization.

In the mid-1970's augmentative releases of adult C. concinnata were made against gypsy moth in eastern Pennsylvania. Although there was an increase in parasitism by the fly the year of release, this was not a practical management technique. It was too expensive to culture them and too little was known about their behavior in the field--the flies dispersed much farther than expected. At the time of these experiments parasitization of native species was not of concern. Now this species would be scrutinized more closely because of its potential impact on nontarget hosts.

- Susan Mahr, University of Wisconsin - Madison

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