Corn rootworms suspected of resistance to Bt corn varieties discovered in Pennsylvania

Posted: August 12, 2014

We have feared that rootworms may develop resistance to Bt varieties in Pennsylvania, and now we have evidence confirming some root worm populations may have done just that.
Figure 1. Roots of a Bt corn variety heavily damaged by rootworms; this corn variety is supposed to resist damage from corn rootworms, but the insects appear to have evolved resistance to Bt

Figure 1. Roots of a Bt corn variety heavily damaged by rootworms; this corn variety is supposed to resist damage from corn rootworms, but the insects appear to have evolved resistance to Bt

For the past few years, folks in Pennsylvania have heard reports from Midwestern states of continuous corn growers struggling to control populations of western corn rootworms that developed resistance to some Bt corn varieties.  Thus far, this problem has occurred elsewhere, with the closest fields with suspected resistance being found in central Michigan and New York north of Ithaca. Unfortunately, this problem may now have found a home in Pennsylvania. 

Last week near Belleville (Mifflin County), I met with a farmer and his consultant to explore three fields that were heavily damaged by western corn rootworms.  The fields had many lodged and goose-necking plants (Figs. 2, 3), and the root ratings I conducted showed that rootworms had removed more than 2-2.5 nodes of roots (on a 3 point scale) from many of the plants (Fig. 1). Further, these fields hosted very large populations of adult beetles, which, of course, a few weeks earlier had been larvae feeding happily on the roots.  These beetles had fed heavily upon leaves (Fig. 4) and had decimated silks on nearly all the ears. There were three corn varieties planted in these fields, two that expressed the YieldGard rootworm trait (Cry3Bb1 toxin) and one expressing one of the Agrisure rootworm trait (mCry3A toxin).  I performed gene checks on the plants to confirm they were producing the appropriate rootworm-targeting toxins and they were. Importantly, all these features are identical to those of fields in the Midwest where resistance to Bt by rootworm populations has been confirmed by University entomologists.  At this point, the fields in Pennsylvania are suspected to have resistant populations of rootworm beetles, but verification of this suspicion will occur over the coming months via work with offspring of the beetles collected from those fields.

Fig. 2.

Fig. 2. Lodged corn in a Bt field in Mifflin County

Fig. 3.

Fig. 3. Goose-neck Bt corn that has suffered from heavy rootworm feeding

Fig. 4.

Fig. 4. Leaf feeding by large populations of rootworm beetles suspected to be resistant to Bt.

The main goal of raising the issue here is to emphasize that this problem can indeed occur in Pennsylvania and that these fields in Mifflin County are unlikely to be alone.  I encourage growers, consultants, and other agricultural professionals to be aware of the possibility of encountering this problem in your part of the state, and know the warning signs detailed above.  Recall that this is a problem for continuous corn; many of our dairies are at risk if they plant corn in the same fields year after year.  

At this time of the year, there is little to do to manage the problem; treating populations of adult beetles is ineffective, so solutions will have to be implemented next spring.  The easiest way to fix this problem, or to prevent it from developing, is to regularly rotate fields from corn to soybeans or alfalfa (or some other non-corn crop for next growing season); try to avoid growing corn continuously in any field for more than two or three years.  Pennsylvania does not have populations of rotational resistant rootworms—that is, rootworm beetles that lay their eggs in soybean fields rather than corn fields and the larvae hatch from eggs and emerge in first-year corn—and we do not expect them to arrive any time soon.  If rotation out of corn is not possible, switch to Bt hybrids that have more than one protein active against rootworms. Finally, ensure that your fields fully comply with refuge requirements.  Planting of refuges is required and can help prevent evolution of resistance. 

The bottom line is that relying on one tactic for too long is a prescription for evolution resistance—just like we have seen with glyphosate-resistant weeds; rootworm beetles are skilled at adapting to consistent management tactics.  Change up your approach to stay one step ahead of this important pest.  To read more about this topic, please see our factsheet on managing potential rootworm resistance to Bt.