The gypsy moth, Lymantria dispar, a forest tree defoliator indigenous to Europe, was introduced to North America approximately 130 years ago. It has spread throughout the Northeastern U.S. forests, south along the Appalachian ridges to West Virginia and Virginia, and west to Ohio, Michigan, Wisconsin, and Northern Illinois. The gypsy moth is not as severe a pest in Europe as it is in the U.S., primarily because many native natural enemies present in European populations do not occur in North American populations. Much of the biological control research on the gypsy moth in the U.S. has been directed toward identifying appropriate natural enemies of this pest and studying the host specificity, methods of introduction, interactions with other natural enemies, and the role of the natural enemies in gypsy moth population dynamics.
Although the host specificity of natural enemies is not an issue in Slovakia where the gypsy moth is native, other studies, including the identification of natural enemies, the ability of natural enemies to suppress gypsy moth outbreaks, and the manipulation and/or application of specific natural enemies (primarily insect pathogens) are important areas of research for forest entomologists in Slovakia. Slovakian and U.S. scientists are collaborating to address some of these questions. Supported by U.S.-Slovak Scientific and Technological Program Project No. 026-95, Drs. J. Maddox and L. Solter (Illinois Natural History Survey and University of Illinois), Dr. M. McManus (USDA Forest Service), and Dr. J. Novotny (Institute of Forestry, Slovakia) are cooperating on studies involving microsporidia, protozoan pathogens of the gypsy moth.
The cooperative research projects are being conducted in both the U.S. and Slovakia. This is an ideal collaboration because field studies, which cannot be conducted in the U.S. due to regulatory constraints, can be conducted in Slovakia, and many of the laboratory studies requiring ultrastructural observations and comparisons of molecular sequences can best be conducted in the U.S. Field projects in Slovakia have included studies of several aspects of gypsy moth-microsporidia interactions. In 1996 and 1997, we surveyed the Slovakian gypsy moth populations to collect isolates of microsporidia; plots were selected and oak trees were banded with burlap for collection of gypsy moths which utilize the bands as resting sites. The surveys yielded several species of microsporidia that are endemic in the gypsy moth populations in three sites. Mortality rates were determined in the laboratory for a promising species of microsporidia, Vairimorpha sp. and were found to reach nearly 90% when early instars were fed high doses of infective spores. Low and high doses fed to later instars produced 45-72% mortality.
In the summer of 1997, the Vairimorpha sp. was used as a bioinsecticide against gypsy moth larvae in the field. The infective spores were suspended in sterile water and were sprayed onto the foliage of young oak trees. The gypsy moth larvae were in the second instar at the time the trees were sprayed; the larvae were collected two weeks later and the causes of mortality determined. The percentage of larvae dying from microsporidiosis or a combination of microsporidiosis and another natural enemy reached 43.9%. These results, and the low mortality rates in older larvae indicate that use of these pathogens as classical biological control agents may be more appropriate than use as microbial insecticides. Evaluation of the spray technology will be continued in 1998 as a method for delivering spores into gypsy moth populations but, for these studies, the horizontal (larva to larva) transmission of the microsporidia, and vertical (from generation to generation) transmission in the field plots will be investigated.
In addition to the field studies, we have surveyed other native forest Lepidoptera from Slovakia for occurrence of gypsy moth microsporidia. The microsporidia do not appear to be shared with other lepidopteran species in the areas of origin, however, spraying the microsporidia may put nontarget species into unusual contact with the infectious spores.
In the U.S., ongoing studies performed under quarantine include taxonomic identifications of the microsporidia collected in Slovakia. These studies involve light microscopy, transmission electron microscopy, and rDNA sequencing. We have determined that there are three distinct species among at least five biotypes of Slovakian gypsy moth microsporidia. The life cycles of the microsporidia and the characteristics of infection in the host were determined for all biotypes. These microsporidia, with one exception, are closely related genetically but the life cycles and major host organs utilized by the parasites differ. One species, Vairimorpha sp., is primarily a fat body parasite that accumulates in enormous numbers in this organ and is probably disseminated into the environment when infected larvae die and decompose. Several other biotypes primarily attack the silk glands and the spores may be disseminated in extruded silk, which other larvae may encounter on silk trails, as well as when cadavers decompose.
Spores of each of the microsporidian biotypes have been mass-produced in the quarantine laboratory at the Illinois Natural History Survey and are stored in liquid nitrogen for future studies and for use in the field studies in Slovakia. The IC50 (concentration of spores that causes infection in 50% of larvae fed spores) and LC50 (concentration of spores that results in the death of 50% of the larvae fed spores), as well as transmission characteristics, are being determined for all the Slovakia biotypes.
Cooperative biological control efforts such as this Slovak/U.S. collaboration illustrate how questions about putative biological control agents can be answered before nonindigenous organisms are introduced, in this case from the area of origin in Slovakia to the U.S. It also demonstrates how researchers in the country of origin of a pest organism can benefit by learning much more about native natural enemies and the possibilities for manipulating natural enemies where more control is needed.
- Leellen Solter (Illinois Natural History Survey), Julius Novotny (Forest Research Institute, Slovak Republic), Joe Maddox (Illinois Natural History Survey), and Michael McManus (USDA Forest Service, Hamden, CT)
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