Thursday, October 26, 2017

Southern Bt Crops: Getting Boxed In

Insects have developed resistance to the older Bt toxins in cotton and corn on a local or regional scale. A quick look at the situation in the southern U.S.A. finds that in the last two years, resistance has been documented over a large geographic area in cotton bollworm/corn earworm (Helicoverpa zea) to the cotton Cry1Ab, Cry1Ac and Cry2Ab2 toxins. In corn, the limited efficacy of Cry1Ab, Cry2Ab2 and Cry1F has slipped from where it was years ago. This year in the mid-south, university personnel are reporting as much earworm damage in Bt corn as in non-Bt corn.

The "new" toxin, Vip3a, is highly effective on bollworm/earworm, and seed companies are putting it into new corn hybrids and cotton varieties alongside suites of the older toxins for which resistance has developed. Yes, readers can already see what is wrong with this picture; once Vip3a hybrids and varieties are widely planted, bollworm/earworm will be selected for successive generations on this toxin that now has only partial protection from the other toxins because they are already compromised. There will be two generations of Vip3a resistance selection on corn, and then another 1-2 generations of selection on cotton.

When bollworm/earworm becomes resistant to Vip3a, cotton will suffer economic damage because bollworm is a major pest. Resistance will be noticed in cotton first, in part because Vip3a toxin expression is lower in cotton than in corn. When Vip3a fails in cotton, growers will then begin to spray their crops 1-3 times per season with the diamide class of insecticides, the most effective class on bollworms. This in turn will select bollworm for resistance to diamides. Cotton production costs will rise and profits will decline. Any new toxins are seven to ten years away, so profitable cotton production depends on keeping bollworm susceptible to Vip3a for as long as possible.

On the other hand, on the vast majority of acres in the southern U.S., corn does not need the protection afforded by Vip3a. (The exceptions would be the few areas where fall armyworm is a pest, and in very small areas where fumonisin levels can increase due to insect damage to ears.) Corn earworm is not a significant pest in field corn; it damages the tips of ears and is not a major contributor to yield loss. This fact, however, has not stopped the seed companies from marketing Vip3a corn as a breakthrough solution to the corn earworm "problem". It is easy to sell technology that results, at least for a few years, in a completely undamaged ear, whether that technology is needed or not.

Unnecessary insect control aside, seed companies have put their most advanced genetics into Vip3a corn, so even without insects in the system these hybrids will probably yield more than older hybrids with compromised Bts or no Bt. Because they can't get advanced genetics in older, non Vip3a hybrids, growers will end up planting Vip3a corn for the yield potential rather than a need for Vip3a.

As a consequence, it is easy to predict that Vip3a corn acres will expand in the south and therefore hasten the demise of bollworm susceptibility in cotton. If this happens, the cotton industry will suffer major losses and corn growers will barely notice. Earworm/bollworm moths resistant to Vip3a (and perhaps the diamide insecticides) will fly north and threaten the sweet corn industry.

The older Bt toxins are failing, and Vip3a stands as the primary means of caterpillar control for some pests. There is already a field collected colony of fall armyworm that is resistant to Vip3a, but as yet bollworm/earworm is susceptible, although cotton varieties with Vip3a did suffer bollworm damage this season (which does not mean resistance).

I have long been a proponent of GM crops and still am. However, the current situation highlights the fact that we are just on a different version of the old Pesticide Treadmill. We are being boxed in; trying to come up with the next great thing before the last great thing loses effectiveness. I hope the next great thing comes along in time.


For a bit of background, older issues of this newsletter have discussed the convergence of Bt toxins in our crops and what it means for resistance evolution.

Is There Still Value in GM Crops? (9/16/16)
Bt Corn and Resistance Clouds (2/6/16)
Shuffling the Deck Chairs in Bt Crops (9/10/16)