Although this caterpillar is attacked by a wide range of native wasp parasitoids, most notably Itoplectis conquisitor, the parasitoids do not generally keep bagworms at acceptable levels in landscapes and nurseries. Bacillus thuringiensis var. kurstaki (Bt) is quite effective against early instars (bags <1" long), with control regularly in the 75-90% range. Applications against later instars are generally less effective, dropping to about 60%.
Field trials conducted in Maryland over the last few years indicate that two entomophagous nematodes, Steinernema carpocapsae and S. feltiae, can be at least as effective as Bt for controlling bagworm. Nematode applications on early instars resulted in 70 and 67% control, respectively, while mortality with Bt was 77%. As expected, these materials were less effective on later instars with only 51% and 58% control, respectively, and 59% by Bt. Adding oil to the nematode solutions seemed to reduce their effectiveness, while addition of the antidesiccant Folicote marginally increased the effectiveness of S. carpocapsae in some trials. The high degree of nematode success may be due to the humid environment created in the bags surrounding the caterpillars and the warm summer temperatures. The trick, of course, is to get the nematode into the bags, before they dry out and die. Applying the nematodes during periods of high relative humidity is critical to this process.
Gill, S. A. 1995. Bagworm Battles. American Nurseryman 182(1): 63-69.