Know Your Friends

Elm Leaf Beetle Parasitoids

The elm leaf beetle came from Europe in the early 1800's, and spread from the eastern United States across North America. Although elms are no longer as numerous as before the ravages of Dutch elm disease, the elm leaf beetle is a major defoliator of elms in urban areas throughout the U.S. There are many natural enemies of elm leaf beetle, including two eulophid wasps in the genus Tetrastichus.

Tetrastichus gallerucae is an exotic parasitoid that attacks elm leaf beetle eggs. It has been introduced into the U.S. several times in attempts to control the elm leaf beetle. The initial importation was made in 1908, when wasps from France were utilized in lab propagation. Field releases of adults from those stocks were made in Massachusetts, New Jersey and Washington, D.C. They failed to establish, so additional importations were made from various European countries in 1911, 1925, and 1932-35. Despite widespread releases in states from Massachusetts to Virginia the parasite failed to survive. T. gallerucae is established in the Midwest--in central Ohio, apparently as the result of releases made there in 1932, and central Missouri--and at a few locations in California, apparently from releases beginning in the late 1970's.

The adult wasp is 1/8" long, with a dark metallic blue thorax and a black abdomen with a faint metallic luster. It is a solitary internal parasite, completing its life cycle in about 17 days. There are several generations each season, usually twice the number of annual generations of its host. It probably overwinters as adults in sheltered places. The females also destroy many eggs by host feeding; this is considered an important factor in reducing the pest population in Europe. In Ohio T. gallerucae appears to control elm leaf beetle under certain circumstances. Inoculative releases in areas where it cannot overwinter well might be useful for beetle suppression.

T. brevistigma, apparently native to the northeastern U.S., attacks elm leaf beetle pupae. Its existence as a parasite of the leaf beetle is somewhat of a puzzle. Its only known host is of European origin, yet the parasite has never been reported from Europe. First described in 1932 in Massachusetts, it is widespread throughout the eastern states and as far west as Indiana and Kentucky. In 1934 it was shipped to California, released, immediately established and became widely distributed and abundant in some areas and during some years, but seems to be unimportant compared with other elm leaf beetle parasitoids introduced there.

T. brevistigma is a gregarious internal parasite. The 1/8" long females lay from 1 to 8 eggs at a time in beetle pupae or prepupae. Each female can lay up to 100 eggs. An average of 12 individuals develop in each host, but up to 37 have been recorded from a single beetle pupa. Larval development can be completed in 9-15 days, but many individuals enter larval diapause until the following spring. The percentage of larvae that will hibernate increases from generation to generation, and practically all the full-grown larvae of the last generation hibernate. They overwinter as full-grown larvae in the pupal skin of their host. These dead beetle pupae are found in the grass and debris surrounding the bases of the elm trees or on the tree trunks, concealed in crevices or under the bark. The parasitoids emerge from the host pupae in June and July to attack the first generation of the beetle as it begins to pupate.

In the northeastern U.S. T. brevistigma commonly parasitizes 50-80% of the beetle pupae in midseason. In areas where parasitization is less, augmentative releases might prove effective. Although rearing techniques have been developed, neither T. brevistigma nor T. gallerucae is currently commercially available.

- Susan Mahr, University of Wisconsin - Madison

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