Earworms/Bollworms in Bt Corn
Earworms (known as bollworm in cotton) have been fairly absent in our system over the past few years. Arguably, at least a small part of this has to be due to widespread plantings of corn hybrids with the VT Double PRO traits and adoption of the newer Agrisure Viptera and Intrasect Leptera traits. Over the past couple years, there have been spots in our state where failures have been reported from VT Double PRO. In research plots in the Carolinas, we have been tracking the efficacy of this trait and have noted a decrease over time (see figure). That has led to earworms breaking through a lot of our VT Double PRO corn this year across the state.
The good news is that corn earworm has been demonstrated time and time again through replicated and published university Extension research to not impact yield in timely planted corn. These results are consistent across the southern US and have been proven recently in the Carolinas. The bad news is that we have also demonstrated that earworms that develop on Bt corn survive as well as those from non-Bt corn (once they pupate). Also, once they mate and lay eggs as adults, they will lay as many viable eggs as those coming off non-Bt corn. Furthermore, earworms that develop and survive from Bt corn will survive better on Bt cotton than those that develop from non-Bt corn. This means that many of the earworms coming off corn now will present a problem for Bt cotton later in the season (see this article for new 2016 cotton recommendations)
Finally, earworm populations are heavier than they’ve been in years past, so we might expect more problems in both cotton and soybeans. The earworm populations seem to be developing off corn a week to 1 1/2 weeks earlier than normal. This, combined with our late cotton and soybean crop will probably contribute to some unique management challenges as the season progresses.