Dr. Chris DiFonzo, Entomologist at Michigan State University and author of the Handy Bt Trait Table, and I got a note from a corn seed dealer a couple of weeks ago concerning the removal of some single toxin Bt corn hybrids from the market after 2019.
Apparently there was a letter circulating from one of the seed companies to this effect, and he wanted to know whether it was true and, if so, why it was being done – some of his customers really like their single toxin hybrids.
It is true, and it is a good and necessary thing. Back when Bt corn was first introduced, most hybrids had only a single toxin for caterpillar pests, and, a few years later, if there was a corn rootworm toxin it was single as well. A few more years down the road, seed companies began selling “pyramids” of toxins; a combination of two or more toxins targeted at a pest.
Not only did this improve efficacy, but it also slowed the rate of resistance development as explained in the following scenario.
If insects with resistance to toxin A were allowed to develop on plants that contained only single toxin A, then most of them would live and pass their genes on to the next generation and resistance to toxin A would evolve rather quickly.
Pyramids were meant to slow the process down because in a pyramid of toxins A and B, insects with resistance to toxin A would still be killed by toxin B (unless they also had resistance to toxin B). Similarly, insects that had resistance to only toxin B would be killed by toxin A.
In each case, the resistance genes would be removed from the population. The chance of an insect having resistance to both toxins A and B was initially quite small (but is not so today now that we have grown Bt corn and cotton for more than 20 years).
Seed companies, realizing the risk of their single toxins being selected generation after generation, soon began cross-licensing their toxins to each other in an attempt to build pyramids as fast as possible.
What has changed is that we now have insect species where resistance to one or two of the toxins in a pyramid is fairly common. If a pyramid is built from toxins A and B, and an insect is now completely resistant to toxin A, then it is only toxin B that can kill it.
So in reality, the insect is being selected for resistance to toxin B alone now that toxin A has no effect. But if resistance to toxin A is not complete, and toxin A still has some effect (not full effect), then toxin A still provides some partial protection to toxin B.
This “partial protection” scenario is where we are today with all of our pyramids of toxins for corn rootworms; every toxin is compromised, but some more than others. In caterpillar control, we are trying to protect Vip3a, the newest toxin in our most modern pyramids.
We are trying to buy time and prevent resistance to all of the toxins in our pyramids. And, since putting a single toxin out there by itself is the fastest way to get resistance to that toxin (and destroy the “partial protection” it still might offer in pyramid hybrids), it makes perfect sense to get the single toxin hybrids off the market as fast as possible.
All of the seed companies have committed to removing all of their single toxin corn hybrids, “as soon as possible”. (Single toxin cotton was removed years ago). Some companies have been more successful than others, but all are trying.
The loss of the few single toxin corn hybrids currently on the market is a small price to pay if their removal will delay resistance a few more years. We are in a tight spot with our Bt toxins in corn and cotton; resistant insects are closing in and we need to extend the life of our current pyramids long enough that the next generation solutions can come online.
If you want to know which toxins are in the corn seed you are buying, the 2019 version of the Handy Bt Trait Table was published earlier this month.