An issue facing corn growers this fall and winter when purchasing corn for next summer centers around the recent announcement of western corn rootworms developing resistance to Cry3Bb1.
The trait is one of the rootworm-control Bt genes that has been incorporated into various corn hybrids including YieldGard VTRW and YieldGard VT Triple, and in the pyramided hybrid SmartStax. This was reported on in July and we addressed it in an article in the C.O.R.N. newsletter from Aug 9-16, Issue 26.
Currently, this resistance has possibly shown up in southern Minnesota, northern Iowa, and northwest Illinois. Reports are that these fields had been in continuous corn for the past 3-4 years, and relied on a single Bt gene, Cry3Bb1, during that time.
While researchers and companies further examine this phenomenon in those states, the question is what should growers in Ohio do IF they plan to use Bt transgenic hybrids for rootworm control.
At this time, we do not see any reason why hybrids containing the Cry3Bb1 gene should not be used. There are no reports of problems in Ohio of significant injury when using these hybrids, and we need to remember our rootworms populations are not as heavy or widespread as in those states to our west.
And with SmartStax, there are two other Bt genes, Cry34/35Ab1, that would control the rootworm larvae.
However, we do see this concern as a good learning experience on what growers should do to limit the potential for resistance to develop:
1) Rotate to soybeans or another nonhost crop;
2) Use a Bt hybrid expressing a different corn rootworm Bt protein than what has been used the previous year if growing continuous corn;
3) Use a pyramided Bt hybrid that expresses multiple Cry proteins targeted against corn rootworm;
4) Use one of the corn rootworm soil insecticides at planting which are still very effective;
5) If rootworm problems have not been that heavy in your area, consider a high rate of a seed treatment (although realize they have not been that effective if rootworm populations are extremely high), and finally
6) Make use of the appropriate refuges!
For effective corn rootworm management, it is important to use a long-term, integrated approach that includes multiple tactics. Too many growers have relied on a single tactic for too many years, and unfortunately, resistance is apparently beginning to emerge.