Macrocentrus grandii, a braconid parasitoid of European corn borer, is native to Europe and Asia. It was one of three exotic parasitoids that became established in North America from 26 species introduced in 1926. This wasp was first released in Massachusetts and became the dominant parasitoid in the east over Lydella thompsoni and Eriborus terebrans, the two other species that became established. In the east parasitization by M. grandii at that time was up to 56%, but much lower in other areas.
Adult wasps are about 3/16 inch long, with a black head, and a yellowish-brown to blackish-brown body. The ovipositor of the female is longer than the rest of the body. After a 3 day preoviposition period, the female deposits her eggs singly in second or third instar corn borer larvae. Larval tunnels with frass and webbing are very attractive to the wasps. The female raises the end of her abdomen, and probes rapidly in the area where the borer has been feeding. The ovipositor is then inserted through the plant material into the borer, with the sheath around the ovipositor forming a loop upwards and backwards. Females deposit 200-300 eggs. Each egg develops into 15-25 embryos (polyembryony). In first generation corn borers the eggs hatch within a few days, but in second generation borers they remain unhatched in the overwintering host larva until the following spring. The larvae in overwintering borers hatch in early April and feed internally through 3 instars. Immediately after the third molt, the parasite larvae emerge from the body of their host through the skin and feed externally until the borer is emptied. Glossy light brown silk cocoons are constructed in an elongate group, to form a 1 inch long cigar-shaped mass. The cocoon group is attached to the shriveled remains of the host. The adults emerge in about 10 days, in late June and July to parasitize the next generation of European corn borer.
M. grandii is most abundant in a band from northeastern Pennsylvania to eastern Virginia. It may parasitize up to 50% of corn borer larvae in these areas, but much less in the Midwest. M. grandii became established in the Midwest during the late 1940's, and was soon considered an important mortality factor of the corn borer. Wasp populations declined, however, in the early 1960's, about the same time the microsporidian pathogen Nosema pyrausta--which affects both the wasp and the borer--became established here. The wasp was detected in only five north central states in a 1987-1990 USDA survey of European corn borer parasitoids. Very high levels of parasitism at a few locations in Wisconsin in the early 1990's suggests the wasp may be locally abundant, even though average populations are quite low.
There can be a dramatic difference in parasitization rates between the first and second corn borer generations. In surveys in Illinois in the early 1980's, parasitism of first generation European corn borer averaged 18%, while parasitism in the second generation was less than 5%. In a study in Michigan, M. grandii was present only in the second generation in 1989, but not at all in 1990. In Wisconsin a few years after that, first generation parasitization up to 65% at some locations, while second generation parasitization was much lower at these same sites.
Today M. grandii is considered a minor mortality factor of the European corn borer in the Midwest, although it may be locally abundant at least in some areas, where it can have a significant impact on borer populations.
- Susan Mahr, University of Wisconsin - Madison
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