Saturday, February 6, 2016

Bt Corn and Resistance Clouds

Cracks small and large are beginning to appear in our Bt (transgenic) corn targeted at controlling insects. To date we have confirmed resistance in corn rootworm to Cry3Bb1 and mCry3a. In the caterpillars we have confirmed resistance to Cry1F in fall armyworm, and likely resistance to Cry1F in southwestern corn borer and western bean cutworm. There was recently a population of cotton bollworm (corn earworm) with elevated levels of tolerance to Cry1Ac reported in Arkansas, and bollworm seems to have been becoming less susceptible to Cry1Ac in the cotton belt for many years.

None of this should be a surprise because transgenic toxins are really no different from traditional insecticides; the more you use them the faster the insects become resistant. What is different is that traditional insecticides last only a few days or weeks and are gone; our transgenic crops express toxins all season and can select every generation of a pest for resistance. And also unlike traditional insecticides, we are planting almost all of the U.S. acres to insect resistant transgenic corn and cotton. In corn around 90% of the acres contain Bt toxins for insect control (whether that protection is needed or not). Amazingly enough, two seed companies are considering introducing insect resistant transgenic soybeans in the mid-south, where we already grow transgenic corn and cotton with the same toxins as planned for soybeans.

Transgenic corn for caterpillar control was introduced in 1996, and at that time we tried to build Insect Resistance Management (IRM) plans to prevent resistance from developing for at least 20 years. That effort was mostly successful, with the exception of fall armyworm and Cry1F; the confirmed failure was documented 2010. Since 1996 the seed companies have put additional toxins in corn and cotton and created pyramids of Bt toxins. A pyramid is when there are two or more toxins targeted at the same pest or pest group. For example, Pioneer Leptra is Cry1Ab + Cry1F + Vip3a, and all of these are targeted at caterpillars. These are the same caterpillar toxins as in Syngenta's Agrisure Viptera 3220 and Agrisure Duracade 5222. A stacked pyramid has pyramids for two different types of pest, and Monsanto's SmartStax is a good example. It has Cry3Bb1 + Cry34/54 targeted at corn rootworm, and Cry1F + Cry1A.105 + Cry2Ab2 targeted at caterpillars. Transgenic seed companies are now cross-licensing their toxins, and this effectively means that insects can be exposed to the same or similar toxins regardless of the company that supplied the seed. More and more acres are being planted with the same toxins regardless of the company from which the seed was purchased.

The idea of a pyramid is that it delays resistance by having multiple toxins present. If an insect is resistant to, say, Cry1F, it will likely be killed by another toxin in the pyramid. But now our pests are beginning to develop resistance to several toxins, and it might be that only one toxin in a pyramid of three toxins still has full efficacy. This would mean that the resistant insects are actually being killed by only one of the three toxins, and at the same time are being selected for resistance to that toxin. Vip3a is the latest toxin to be introduced to pyramids for caterpillar control and it works very well, at least for now. But the most modern Bt corn hybrids from DuPont Pioneer and Syngenta already have Vip3a, and Monsanto's next generation of Bt corn will have Vip3a as well. (And it is now in some types of Bt cotton.)

I have written all of this to say that insects are now showing early signs that our Bt technologies are not the panacea that we once hoped they were. Resistance is a natural result of the use and over-use of a technology, and we should not be surprised in the coming months and years to hear that resistance has been confirmed to this or that Bt toxin. The important thing will be to let someone know if you see unexpected insect injury in your crops. We can make collections of insects and send them for resistance screening. It is vital to know where the pockets of resistance are and how far they have spread. In fact, depending how early the resistance was detected and on the pest and extent of spread of resistance alleles, it may be possible to wipe out a local resistance episode. So don't hesitate to pick up the phone and call your local Extension Entomologist or IPM Agent, crop consultant or seed company representative if you see unusual damage in your Bt crops.