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.

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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)

Fumonisin Levels and Insect Damage in Corn

I am not smart enough to be a Plant Pathologist, and in fact had two courses in it in college and still don't understand it. The classic "disease triangle" taught in pathology says that disease occurs when there is a pathogen, susceptible host and conducive environment. This year we seem to have had a happy triangle for Fusarium species, the causative agents of fumonisins.

Not much is known locally about how these fungi interact with our corn, but it is thought that drought stress followed by warm, wet weather, especially at flowering, favor the fungi. Being just an entomologist, I tend to think there is a baseline risk for significant fungal infection based on the susceptibility of the host (hybrid genetics) and environmental conditions. Without insects in the system there will be a given level of fungal growth and fumonisin creation. In my simplistic entomologist's picture, the baseline level is what it is and can vary from year to year, but insect damage can add to this level by opening wounds on the ear and/or by insects carrying fungal spores into the ear.

Dr. Ed Bynum and I did some work at Lubbock in 2012 that looked at the amount of fumonisin in ears with three different levels of insect damage, and more fumonisin was found with higher levels of insect damage. This was one hybrid of non-Bt corn that we sprayed with different timings of insecticide so as to get the three damage levels.

Figure 1. Type of ear damage and fumonisin levels associated with that damage, 2012.

This year there is a need to try and determine to what extent insect damage might be contributing to fumonisin levels, but this is not easy to do unless the hybrids have the same genetic background (inherent susceptibility) and are grown in the same field under the same conditions. One seed company has a small plot field trial near Ralls, and they were kind enough to allow me to sample ears from their new hybrid that contains Vip3a and other toxins, and an older Bt type that has fewer toxins but still the same genetic background as the new type of corn. This is a fair comparison for determining the role of insects. The older type of Bt corn averaged 3.6 damaged kernels per ear, while the new corn with Vip3a was essentially undamaged. Even the silks on the new type of corn were intact. The photos below represent what I saw in the field today.

Figure 2. New hybrid with Vip3a and other toxins (top), and older Bt with two toxins (bottom). The new hybrid was essentially without insect damage. In the older hybrid the insect damage was only at the tip, but fungal growth could proceed through much of the ear. 

The same photo as above, but rendered in an infrared simulation that highlights the kernels damaged by fungi. The grain from these ears was sent for laboratory analysis of fumonisin content, and the relatively undamaged ears on the top of the photo had an average of 6.0 ppm fumonisin. The ears on the bottom row had an average of 208 ppm. 

It is common on the High Plains for nearly every ear of corn to have corn earworm damage, and this year was no different and not significantly worse. In the opinion of this entomologist, the problems we are having this year are primarily due to environmental conditions that favored Fusarium. Having said that, I have worked with Vip3a corn for six years, and in all that time have seen only two live caterpillars in thousands of ears examined. Vip3a corn is essentially bulletproof for now, and if the goal is to reduce caterpillar damage then this type of corn is the way to go. Of course it is more expensive than older Bt technologies. All of the seed companies put other Bt toxins in with Vip3a. Pioneer sells their Vip3a corn as Optimum Leptra or AcreMax Leptra, Monsanto is now beginning commercial sales for 2018 as Trecepta, and Syngenta calls it Agrisure Viptera or Agrisure Duracade 5222. This is not to say that these hybrids won't have fumonisin problems; the inherent susceptibility might be more or less. It is to say that they will have less insect damage, which our data suggest ultimately plays a role in fumonisin levels.

Update on 10/11/17: Erin Louise Bowers did her Ph.D. dissertation on the benefits of transgenic corn in reducing fumonisin levels. She found that Cry1Ab +Vip3a corn had lower fumonisin levels that other types of Bt corn and non-Bt corn. The work is here.