Soybean Podworm (a.k.a. Corn earworm) Damage on Soybeans
Figure 1. Soybean podworm damage of seed wall and in the process of feeding the bean. Photo by R.T. Villanueva.
Helicoverpa zea (also known as the corn earworm, cotton bollworm, and the tomato fruitworm) is a well-known pest of corn, cotton, tomato, hemp, and other crops including soybeans, where it can be a challenging pest, especially in the southern United States. Their preference for the reproductive structures of the soybeans can impact yield, particularly when infestations occur during early reproductive plant growth stages.
Description of insect and damage
Figure 2. Holes caused by the feeding of soybean pods. Photo by R.T. Villanueva.
Although some soybean fields have been already harvested, there were soybean fields that had some maturing pods, or double-crop fields had developing pods by mid-September when I observed caterpillars damaging pods in research plots at the University of Kentucky Research and Education Center. Even if not reported in commercial fields this year, the soybean podworm might have been feeding in maturing soybean pods without causing significant damage. Soybean podworm adults prefer to oviposit on hairy leaves even when larval development is best on smooth (glabrous) leaves (Lambert et al. 1992). Leaf hair morphology can also impact larval feeding damage, which is reduced on soybean with sharp-tipped pubescence compared with those with blunt-tipped pubescence, however, there is not a commercial cultivar developed for this purpose. In soybeans, the most serious injuries happen when late larval stages (fifth or sixth instars) coincide with pods that are developing, and they eat away the pod wall and consume the seed (Figure 1); that potentially may reduce yields.
Although, Figures 1 and 2 were taken on soybean pods taken from a research plot that is near corn planted late with purpose to have infestation of corn earworm; in some commercial soybean fields this happened, but I did not see or hear any reports. Usually, fifth (17.9 mm =0.7 in) and sixth (25 mm =1 in) instar larvae may start to do damage pods causing holes (Figure 2) on them. These large caterpillars are voracious and if outbreaks occur, they reduce soybean yields. Feeding by small caterpillars on terminals, flowers, and small pods does not reduce yield. However, occasionally large caterpillars will cause severe defoliation, but this damage rarely reduces yield.
Management
Planting time is the best resource to avoid soybean podworm damage. Usually, this pest is most frequently found in late planted fields. Double-crop soybeans may be more affected than full season. Proper planting is important as leaves may cover the plant canopy before bloom thus pods may be protected. Control measures to reduce corn earworm damages should be conducted when an average of one small worm per foot of row is detected. The effects of insecticides for late larval stages is reduced, thus insecticide applications should be completed for small size larvae. Insecticide treatment to reduce corn earworm damage is recommended if 5 to 10% or more of the soybean pods exhibit feeding damage from soybean podworm larvae.
