Worms in Your Ears?
Posted: August 31, 2016
Penn State’s PestWatch network, which traps for moths whose larvae (caterpillars) are pests of sweet corn, has detected in the past few weeks a strong increase in corn earworm moths. At the same time, as farmers explore their maturing corn fields, some are calling our extension offices with concerns about corn earworms in their fields. So, I thought I would take some time to address these caterpillars in ears of field corn and their relevance for growers.
Most of the caterpillars that folks are finding are indeed corn earworm. This insect species does not overwinter well in PA and our infestations of corn earworm larvae usually result from eggs laid by adult moths that fly here from southern states. In field corn, these caterpillars colonize ears every year, but their populations tend to be patchy. Most Bt corn hybrids offer some protection against corn earworm. In sweet corn, some Bt varieties are available, but for non-Bt varieties common recommendations are to spray every 6-14 days (or even less) when adult moths are active and laying eggs to kill young larvae before they enter the ear.
The frequency of sprays depends on populations of adult moths estimated from PestWatch. For field corn, this approach is impractical because of the large acreage involved. As a result, development of Bt traits active against corn earworm has been a benefit for controlling a pest that was not previously controlled. Nevertheless, farmers tend to be disappointed when the find worms in their field corn. My advice is usually to ignore the damage because it is unlikely to be causing a significant yield hit. Research that my lab conducted from 2010-2012 clearly indicated that corn earworm populations across PA were low and the caterpillars were unlikely to be causing meaningful yield loss, even in non-Bt corn. In other words, farmers that targeted these caterpillar populations with insecticidal sprays would spend far more money on the material and application than they would gain in protected yield. I believe this situation persist today; corn earworm in PA likely remains only a minor, possibly annoying, pest species.
If the caterpillars in your corn ears are not corn earworm, you may have another problem. Colleagues in Michigan, Ohio, Indiana, New York, and Ontario have been suffering from western bean cutworm, which also infests corn ears. For the past six years, I led an effort to trap for this moth species in PA, which first arrived here around 2009. But across the state, we captured relatively few moths and found only a handful of caterpillars, so we discontinued our monitoring efforts. To our west and north, however, western bean cutworm has been causing economic damage to ears, even in Bt hybrids meant to control it (e.g., the Herculex trait, Cry1F protein). Particularly if you are in the northern part of the state, I would encourage you to determine if your ear caterpillars are western bean cutworm or just corn earworm. The identity of the species matters a great deal and may explain the level of damage. Western bean cutworm lacks the strong longitudinal stripes of corn earworm and has an obvious brown collar (called its pronotum) behind it head that has three small stripes that parallel the body (Fig. 1).
Fig. 1. Western bean cutworm caterpillar, showing the distinctive longitudinal lines in its collar (or pronotum) behind its head.