Corn earworms have been found on both sorghum and soybeans in Kansas this summer, and the infestations will continue for the next few weeks. Earworms are often called "headworms" when feeding on sorghum or "podworms" when feeding on soybeans. The main impact on yield is when the larvae infest the heads of sorghum or the pods of soybeans. In either crop they can cause significant yield reductions very quickly as they are feeding directly on the marketable product.
Corn earworm larvae need to be detected while they are still small (less than a half-inch long), before they do much feeding. Less will be gained by treating older, larger larvae because they are nearing the end of their feeding period. Sorghum will be vulnerable until it reaches the dough stage and soybean pods are vulnerable until the seeds begin to harden.
Field sampling should begin as soon as sorghum starts heading. At several points in the field, bend over a few sorghum heads into a clean, white 3-quart or 1-gallon bucket and shake them vigorously against the sides to determine how many worms are present per sorghum head. Keep track of how many heads you have shaken into the bucket. One to two worms per head can result in approximately 5 to 10 percent yield loss. The decision to treat should balance the expected loss of yield and crop value against treatment cost and the amount of damage that can be prevented.
Begin sampling soybeans once they have started setting pods. At a minimum of 10 locations in a field, bend over one-foot sections of row, shaking insects onto a cloth spread on the ground on both sides of the row. Then calculate the average number of larvae per row-foot. In soybeans, control measures generally should be implemented when an average of one small worm per foot of row is detected.
Insecticides labeled for this application all seem to work well provided they are applied with enough water to get adequate coverage. Most insecticides will provide 10 to 14 days of residual activity, maybe even a bit more, so continue scouting until the sorghum or soybean seed have become too tough to be attractive to the worms.
For more information relative to sorghum headworm or soybean podworm management refer to K-State’s Sorghum Insect Management Guide, 2011, MF-742, available at your local county Extension office or at: http://www.ksre.ksu.edu/library/ENTML2/Mf742.pdf
Also see K-State’s Soybean Insect Management Guide, 2011, MF-743: http://www.ksre.ksu.edu/library/ENTML2/Mf743.pdf