While studying the parasitization of adult alfalfa weevils by Microctonus aethiopoides over the last six years, Wisconsin researchers discovered red larval mites, later identified as Trombidium newelli, attacking a number of weevils. This species was previously recorded from alfalfa weevil in Ohio and has recently been discovered on bean leaf beetles on soybeans in Iowa, but had never been recorded from Wisconsin.
T. newelli larvae are found under the wing covers on the surface of weevils' abdomen. The mite larvae are equipped with powerful mouthparts that are used to tear tissues and hold on firmly to the host. The mites usually insert their mouthparts in the membranous areas between the abdominal segments. They extract nutrients from the host blood while living externally. The mite larvae remain attached to the hosts until fully engorged, then drop off to complete their development.
T. newelli appears to have only one generation per season, and is mainly restricted to the summer weevil populations. The seasonal activity of the mite larvae is well synchronized with emergence of the new weevil adults (summer population) in late June, but they only infest weevils for a short time. The mites were found in the field for less than 50 days each year. Both adult female and male weevils are parasitized, with an average of 2.4 mites per weevil. The parasitism rate averages 16%, but varies from year to year.
The mite larvae appear to cause considerable damage to the weevils while feeding, which could impair weevils' reproduction and survival, increase their susceptibility to diseases, and hasten their death. T. newelli is a good addition to the limited number of natural enemies attacking adult alfalfa weevils, increasing the biotic stress on the weevil populations for improved control. Further study of the biology of the adult mite is needed to determine its role in the alfalfa ecosystem.
- AbdulAziz Mohamed and Dave Hogg, University of Wisconsin - Madison
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