Caption: European corn borer larva feeding in the whorl of a corn plant.
Corn varieties modified for herbicide tolerance and insecticidal proteins can be extremely difficult to sort through when deciding which corn hybrids to purchase for the next season. If purchasing a corn variety tolerant to both glufosinate and glyphosate, pay special attention to whether the refuge contains a variety with tolerance to both or just one of the herbicides. Bt corn produces an insecticidal protein or group of proteins that can effectively control larvae of common corn insect pests. However, just like an herbicide may not be effective on some weed species, the specific Bt trait varies in its ability to control certain insect pests.
In order for the Bt toxin to eliminate an insect pest it must be ingested. For moth and beetle pests, only the larval and adult stages where the insect is actively feeding will be controlled. In Pennsylvania, the potential insect targets for Bt- traited corn include: black cutworm, corn earworm, European corn borer, fall armyworm, true armyworm, and corn rootworm. The refuge requirement, where a certain percentage of a non-Bt traited variety is included in the bag or in the planter, helps to manage for resistance to Bt toxins. However, Western corn rootworm resistance to Bt toxins is a serious concern particularly for continuous corn fields. There are several tactics that can be used in combination to prevent resistance. To read more about managing resistance in Pennsylvania, please read the following article, "Managing Western Corn Rootworm Resistance to Bt on the fringe". One tactic to consider is rotating Bt toxins.
Rotating Bt toxins may seem like an impossible task but there is a resource to help select varieties for your rotation. Entomologists from Michigan State University, Texas A & M, and Ohio State publish “The Handy Bt Trait Table” at least once a year. The latest version, updated on March 15, 2017, is available on Michigan State’s website. This is an excellent resource with both the herbicide tolerance and Bt traits completely outlined for Agrisure, Herculex, Optimum, Yieldgard, Genuity, and other trait families. For each product, the trait, a list of insects controlled, herbicide tolerance, and refuge requirement are provided on the second page. Dr. DiFonzo has added a new column in this version which alerts the reader to whether or not the trait has been found to be ineffective in any geographic location. There is a separate list of references and locations for each product and which insect is no longer effectively controlled. The proteins/toxins that are effective in managing Western corn rootworm are eCry3.1Ab, mCry3A, Cry34Ab1/Cry35Ab1, and Cry3Bb1. With the help of the Handy Bt Trait table, it is possible to design a rotation for the previously mentioned toxins as part of a resistance management plan.