Cotesia marginiventris (armyworm parasitoid)
Blake Layton (Mississippi State University) and Scott Stewart (University of Tennessee), Department of Entomology and Plant Pathology
This small parasitic wasp has no common name, but it still often plays an important role in helping suppress beet armyworm populations. In several studies from Georgia to Mississippi, the percentage of beet armyworm larvae collected from cotton fields that were parasitized by Cotesia ranged from 17-83%. These levels of parasitism can have a tremendous impact on the overall population development of a pest like beet armyworm. Of course, Cotesia is not the only beneficial insect that is important in suppressing beet armyworm. It is mortality caused by a complex of predators and parasites that helps to maintain beet armyworm populations below damaging levels. Thus, the intensive use of broad spectrum insecticides may trigger outbreaks of this pest.
The adult wasp is so small and inconspicuous that it is rarely seen by scouts. It is about 1/8 inch long and has a dark colored body. The female wasp inserts her eggs inside the body of the caterpillar using her stinger-like ovipositor. The white, maggot-like larvae also are seldom seen because they spend their life inside the body of the caterpillar host. This larval period lasts approximately 6 days. Occasionally a scout will encounter a mature larva after it has exited the caterpillar and before it has pupated. It is the pupal stage of this parasite that is most often seen by scouts. Cotesia pupates in an oval shaped white silken cocoon that is approximately 1/8th inch long. These are often found near an area where the host larva was feeding.
In addition to beet armyworm, Cotesia also attacks a wide range of other caterpillars such as yellowstriped armyworm, fall armyworm, tobacco budworm, and bollworm. However, beet armyworms seem to be the most favored host, possibly because their habit of remaining clustered together for several days after hatching makes them easy targets.