An Update on Pennsylvania Populations of Corn Rootworm Suspected of Resistance to Bt
Posted: September 2, 2014
Perhaps you will recall that in early August we reported that we found corn fields in Pennsylvania that appear to have suffered damage from populations of western corn rootworms that may have developed resistance to some Bt corn varieties. Details of our initial finds can be found here and the objective of this article is to provide an update.
Since reporting on the first suspect fields that we encountered near Belleville (Mifflin County), we have also visited suspicious fields in Cumberland and Centre Counties, all of which had many lodged and goose-necking plants, root damage ratings around 2-2.5 (on a 3 point scale), an abundance of adult western corn rootworms, and obvious damage (leaf scraping, silk clipping) cause by these large populations of adults.& The fields expressing these symptoms were in corn for 3-10 years and planted with corn varieties that expressed one Bt toxin targeting rootworms, either the YieldGard rootworm trait (Cry3Bb1 toxin) or the older of the two Agrisure rootworm traits (mCry3A toxin). I suspect that there are other fields in Pennsylvania that have suffered from this same problem, so as harvest season approaches, be on the look out for continuous corn fields with similar symptoms. Feel free to contact me if you find suspicious fields.
As I mentioned, these fields are suspected of having populations of rootworms resistant to Bt. Confirming whether or not the rootworm populations are resistant will be determined next year some time after a bunch of lab work. But whether or not the beetle populations are resistant is somewhat of an academic and regulatory question; the bottom line is that fields are experiencing heavy root damage, the rootworm populations are very large, and, if beetles are not resistant yet, they will be in coming years if changes are not implemented. From a grower’s perspective, the main goal should be to manage beetle populations to squash the problem and prevent it from reoccurring. The best approach to dealing with large rootworm populations in Pennsylvania is to regularly rotate fields from corn to soybeans or alfalfa (or some other non-corn crop the next growing season), and do your best to avoid growing corn continuously in any field for more than three years. If rotation out of corn is not possible, switch to Bt hybrids that have more than one protein active against rootworm. Finally, ensure that your fields fully comply with refuge requirements. Planting of refuges is required and can help prevent evolution of resistance. As I have mentioned previously, relying on one tactic for too long is the best way to develop resistance, so alter your management approach to stay one step ahead. To read more about this topic, please see our fact sheet on managing potential rootworm resistance to Bt.