For folks growing continuous corn, western corn rootworm is a serious pest that needs regular attention. Most growers in Pennsylvania seem to be aware of the ongoing struggle in the Midwest with populations of western corn rootworm that are resistant to some varieties of Bt corn. Similar reports of suspected resistance have emerged from Michigan and New York. This resistance issue continues to be serious and we do not want it to develop in Pennsylvania or the eastern corn regions. This issue has been addressed before, and as a reminder, rotating crops is the best approach for managing root worms.
In response to this resistance threat, many growers have considered putting insecticides over the top of Bt-rootworm seed. Growers should be cautioned that combining these tactics will not necessarily increase yield. In fact, a recently released research paper from the University of Illinois and one last year from Iowa State University indicate that adding soil insecticides to Bt seed tend to improve root protection a little but do not increase yield. Together, the two research papers addressed single-trait hybrids, hybrids with more than one Bt gene, and refuge-in-a-bag, and insecticides did not provide a yield benefit in any case.
So when you are planting corn this spring, think twice prior to putting insecticides over the top of your Bt.
(The two papers are: 1) Petzold–Maxwell et al. 2013. Effect of Bt maize and soil insecticides on yield, injury, and rootworm survival: implications for resistance management. Journal of Economic Entomology 106: 1941-1951; and 2) Tinsley et al. 2014. Evaluating multiple approaches for managing western corn rootworm larvae with seed blends. Journal of Applied Entomology. I can share these with you if you're interested, just contact me.)