In last week's issue of the Bulletin (no. 14, June 29, 2001), I discussed thistle caterpillars (the adults are painted lady butterflies) and whitelined sphinx caterpillars being found in some crop fields in Illinois. Wouldn't you know it, others have found these odd creatures, too. John Obermeyer, IPM specialist at Purdue University, is aware that sphinx caterpillars have been found in a couple of counties in Indiana, and I read an article about sphinx caterpillars in a newspaper from Arkansas. Neither John nor Arkansas entomologist Don Johnson identified the species. In fact, I am not certain about the species found in Illinois either; I used my occasionally questionable taxonomic skills to pinpoint the whitelined sphinx as the culprit. However, my identification has not been verified. If we learn the species, we'll let you know.|
In virtually all instances where the whitelined sphinx (tentative ID) caterpillars have been found, they were feeding primarily on weedspurslane in Indiana, cutleaf evening primrose in Arkansas, and a couple of species of weeds in Illinois. So let 'em chomp! Their numbers are intimidating, but no one has detected significant feeding on either corn or soybeans.
The thistle caterpillar may be another matter. Dave Harms, a private consultant in northern Illinois, learned about a field of soybeans in DeKalb County that had spotty infestations of these spiny caterpillars. They had completely defoliated Canada thistles in the field (a good thing) and were beginning to work vigorously on the soybeans (not a good thing). Dave indicated that defoliation in some areas within the field was extensive enough that spot treatments might be necessary.
The economic thresholds for thistle caterpillar in soybeans are the same as they are for all defoliators of soybeanstreatment may be warranted when defoliation reaches 30% before bloom and 25% between bloom and pod fill. However, these thresholds should be adjusted upward (very likely) or downward according to the value of soybeans. If an insecticide is needed for control of thistle caterpillars, consider *Ambush 2E at 3.2 to 6.4 oz per acre; *Pounce 3.2EC at 2 to 4 oz per acre; Sevin XLR Plus at 1-1/2 qt per acre; or *Warrior at 1.92 to 3.2 oz per acre. Use of products preceded with an asterisk is restricted to certified applicators.--Kevin Steffey