There are several parasitoids that attack the alfalfa weevil in the Midwest. Unlike the other weevil parasitoids, the braconid wasp Microctonus aethiopoides (= aethiops) is the only one that attacks adult weevils. It was first imported into the United States from Europe for use against the sweetclover weevil. It was later determined that there are several strains of the wasp that are very specific for different weevil species, and not all strains attack the alfalfa weevil. From many releases made by the USDA at various locations from 1957 through 1988, and perhaps from other accidental introductions, it has become well established throughout the alfalfa-growing areas of the upper Midwest and northeastern United States. M. aethiopoides is considered one of the most important parasitoids responsible for maintaining alfalfa weevil at or below economic levels.
The tiny, stingless M. aethiopoides wasps are about 1/8 inch long. Females are reddish brown and males are black in color. The females will lay eggs only in moving adult weevils. The parasitoid takes a position behind the weevil and inserts her ovipositor into the membranous area at the tip of the weevil's abdomen which is exposed only while the weevil is in motion. M. aethiopoides lays a single egg directly inside the body cavity of the weevil. The larva that hatches from the egg then feeds and completes its development internally in 22-26 days. There are no external signs of parasitism until the larva is fully grown and it forces its way out of the back end of the weevil. The wasp larva spins a whitish silken cocoon and pupates in the soil or under debris. An adult wasp emerges 6-9 days later. M. aethiopoides overwinters as first instar larvae inside hibernating weevils.
M. aethiopoides has two generations per year that are well synchronized with the adult weevil's activity. The first generation females attack and lay eggs in the surviving overwintered weevil adults (spring population) that are actively laying eggs in mid-May. The second generation wasps attack newly emerged weevil adults (summer population) in late June. Parasitism rates of alfalfa weevil adults vary from year to year; in Wisconsin parasitism averages 40 and 52% for the spring and summer weevil populations, respectively.
The effect of M. aethiopoides on the alfalfa weevil population results not only from death of the adult weevils themselves, but also by reducing the number of eggs a female weevil lays. Parasitized alfalfa weevils cease oviposition within three days; male weevils are castrated shortly after parasitization. Although the parasitized summer generation weevils will survive until the following spring, they are unable to reproduce and hence contribute nothing to the alfalfa weevil population. The impact of M. aethiopoides as a biological control agent of alfalfa weevil is significant because of this sterilization. A female alfalfa weevil deposits an average of 1350 eggs prior to the first alfalfa cutting, so each successful parasitization eliminates the potential damage of many alfalfa weevil larvae.
It is important to conserve these parasitoids (and other parasitoids of alfalfa weevil, such as Bathyplectes spp.) by using agricultural practices that are not detrimental to the parasitoids. Effective conservation can be achieved by using degree days for timing sprays to avoid insecticide applications during the periods when the adult parasitoids are active. Leaving uncut "refuge" areas of alfalfa allows parasitized weevils to survive. These and other cultural practices can enhance the beneficial activities M. aethiopoides provides in biological control of the alfalfa weevil.
- AbdulAziz Mohamed, University of Wisconsin- Madison
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