Biological control

Introduction and spread of natural enemies

Although natural enemies have spread westward along with the gypsy moth, it was unclear whether spread of specialist enemies would be slowed in Wisconsin’s isolated infestation. In 1996 and 1997, the Department of Natural Resources conducted a survey of parasitoids and diseases already present. Following the survey, researchers introduced three species to the area to supplement natural spread. Results of the survey and the sustainability of the introduced natural enemies are summarized below.

Survey results. Entomophoga maimaiga was found at one site in the Door Peninsula; nucleopolyhedrosis virus (NPV) and Cotesia melanoscela were present at a site near the southern end of Green Bay; dipteran parasitoids were present throughout the range of gypsy moth; and Ooencyrtus kuvanae was present at five sites in the base of the Door Peninsula.

Supplementing natural populations. The DNR used three criteria to evaluate candidate species: does it have a high degree of specificity to the gypsy moth, is it able to establish in low-density gypsy moth populations and survive in our climate, and is it readily available. Three species (Entomophaga maimaiga, Ooencyrtus kuvanae, and Cotesia melanoscela) fit the criteria and were introduced in areas of Wisconsin where gypsy moth had been established for 4 years. The following discussion describes release methods, subsequent spread and recovery, and recommendations for each species.

• Entomophaga maimaiga— This fungus was introduced using ground up cadavers of infected gypsy moth larvae, a source that was expected to provide a high concentration of spores. The spores were spread in September, October, and November. In subsequent surveys, this pathogen was recovered at approximately 50% of the sites. Recovery appeared to be influenced by rainfall.
  
  This species is a good choice for introductions into newly established and isolated populations of gypsy moth. As of 2000, E. maimaiga has moved into central Wisconsin. Although its spread westward is primarily the result of natural movement, the DNR releases appear to have been beneficial in accelerating its establishment. Natural spread will likely play a larger role during wet years now that the gypsy moth is more established in eastern Wisconsin.

• Ooencyrtus kuvanae—This tiny wasp was collected off infested gypsy moth egg masses from wild populations in Michigan and released in Wisconsin sites in early fall. O. kuvanae were recovered at about 35% of the release sites.
  
  The spread of Ooencyrtus was not greatly aided through introductions. This species has spread rapidly on its own, though. In fact, most populations found during surveys were not associated with introductions. However, this species is easy for the public to find when gypsy moth is at moderate levels, making it a good ambassador species for promoting the concept of biological control. Ooencyrtus is easy to obtain, inexpensive, and fairly specific to gypsy moth when collected from wild populations.

• Cotesia melanoscela—Captive-bred cocoons of these wasps were released at two sites in late fall. The timing was chosen to reduce mortality from parasitism. Only 62% emerged the following spring and no adults have since been collected.
  
  The success of Cotesia introductions is difficult to measure using the burlap band survey technique. However, searches of young larvae at release sites also failed to show evidence of this species. Pest managers might consider the ability to recover a species when selecting a natural enemy as a lack of results could present a problem for maintaining support for a biological control program.

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Native natural enemies

Deer mice—The most important native natural enemy of gypsy moth is the white-footed mouse or deer mouse. This cute rodent relishes gypsy moth pupae and will also attack the large caterpillars, skinning and gutting them before feasting. Deer mice can have a dramatic effect on the growth of gypsy moth populations. Evidence of their effectiveness can be seen by comparing defoliation of oak trees growing in lawns to those growing in a woodland setting with shrubbery, logs and other cover for mice. Mice avoid crossing exposed areas such as lawns, so caterpillars on those trees are protected and their population soars.

Insects—Beetles and other invertebrates can be important predators on the gypsy moth. The big, fast ground beetles kill many caterpillars that they find in the tree tops, on the trunks, or on the ground. Stinkbugs inject a digestive fluid into the caterpillars that turns their tissues into a soup which the stinkbug then sucks up. And though they appear frail, daddy long-legs are predators with a poisonous bite and kill many pupae (they pose no threat to humans).

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