Bt Resistance: Why We Can Expect More Going Forward

    2016 has been a challenging year for our Bt crops. Cotton bollworms did an unusually high amount of damage in many fields of Bt cotton, and corn earworms (which are bollworms by another name) caused a significant amount of damage to corn crops from Texas through Kansas.

    Western bean cutworm caused severe damage in fields of Cry1F corn in the Midwest and Canada where once the toxin provided a reasonable level of control. Fall armyworm is known to be resistant to Cry1F corn in parts of the country. Corn rootworm is resistant to toxins that once did a good job of control.

    One question this fall is whether we have resistance to our Bt toxins targeted at caterpillars and, if so, how far it has spread. Field observations suggest that we do have resistance, but we will have to wait for the results of the laboratory tests on the progeny of the insects collected from the field.

    We don’t have a magic genetic test to detect resistance, so we do things the old fashioned way by crossing field collected insects with laboratory insects and seeing how their offspring survive known doses of Bt toxins as compared to progeny from a colony we know to be susceptible to the toxins.

    This article is not about whether we have resistance, it is about why we will have more resistance. When Bt crops were originally registered and deployed some 20 years ago, the seed companies each had their own unique toxins that worked more or less well on specific pests. Effectively the percentage of the pest insect population exposed to any particular toxin depended to a great extent on the market share held by each company.

    Over time, however, seed companies began licensing their toxins to their competitors. In addition to financial gain there was a good reason for this; two or three different toxins were far better than one for delaying resistance.

    If an insect had an allele to survive on toxin 1, it probably did not have different alleles to survive on toxins 2 and 3. The insect would be killed and its allele to survive toxin 1 would die with it and not be passed to the next generation.

    This strategy of multiple toxins targeted at the same pest (a pyramid of toxins) was successfully employed when corn rootworm in the Midwest became resistant to Cry3Bb1; the answer was to make plants that expressed both Cry3Bb1 (from company A) and Cry34/35 (from company B).

    Rootworms resistant to Cry3Bb1 were still exposed to Cry34/35 and many of them died. However, because they were already resistant to one toxin they were really only being challenged by the remaining effective toxin, so they were back to having to overcome one toxin and not two.

    Astute readers will note that we have four toxins for corn rootworm, so why not add one or both of the other two? The answer is cross resistance; rootworms that are resistant to Cry3Bb1 are also resistant to mCry3a, even if their ancestors never encountered mCry3a. Researchers in Iowa have recently confirmed resistance to the fourth toxin, eCry3.1Ab.

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    A good article on this problem is here, and it says, “Cry3Bb1, mCry3A and eCry3.1Ab all appear fairly similar to the rootworm, and resistance to one is likely to confer resistance to the other two.”

    The example above illustrates that there is a finite limit to the addition of new Cry toxins and, because companies are licensing their technologies to their competitors, essentially our whole arsenal of Bt toxins is being planted on the vast majority of our corn and cotton acres.

    On a national level we are effectively selecting several generations of insects, even on different crops, on the same or similar toxins. (Corn rootworm is only on corn, but many of the caterpillar species infest both crops.)

    If you want to see an example of cross licensing of toxins, look at Chris DiFonzo’s Handy Bt Trait Table for corn. For each company, the products listed toward the bottom of their offerings are the newer types of Bt. Regardless of company they all look pretty much the same. (Cry1A.105 is just a synthetic stack of Cry1Ab, Cry1F and Cry1Ac. Cry1F and Cry1Ac are also used in cotton.)

    The newest silver bullet is Vip3a for caterpillars. It is fairly high dose and does a good job of controlling many species. In their latest generation of Bt corn and cotton, all of the seed companies are now adding Vip3a as a pyramid with older toxins. Once again the insects will have adapted, or partially adapted, to the older toxins, so selection for resistance will be on Vip3a.

    There does not seem to be a way out of the box with corn rootworm toxins, and increasingly we are relying on Vip3a to protect yield while the other caterpillar toxins are failing. Cry toxins had a good run and will hang on for a while longer, but the era of the Cry toxin seems to be ending.

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