Bt Corn Seed Selection in Light of Resistance in Corn Rootworm

By Patrick Porter and Ed Bynum, Extension Entomologists in Lubbock and Amarillo, respectively. 

The August 25th edition of this newsletter discussed how our mCry3a Bt corn was no longer able to control western corn rootworm in an area from Hart, Texas, north to the top of the Panhandle. In that newsletter, even though we were seeing all of the classic signs of resistance, we used the term “probable resistance”, only because our laboratory assays on field collected beetles will not be completed until next year. 

In addition to mCry3a, that newsletter suggested that since there is cross resistance between all of the Cry3-type toxins (mCry3a, eCry3.1Ab and Cry3Bb1), none of these toxins could be expected to provide good control of western corn rootworm. Dr. Aaron Gassmann at Iowa State University, a leading authority on corn rootworm resistance, said, “Cry3Bb1, mCry3A, and eCry3.1Ab all appear fairly similar to the rootworm. Resistance to one is likely to confer resistance to the other two.” 

As seed purchase decisions are made for next year’s growing season, it is time to put the cards on the table and discuss options for corn rootworm management.

By far the best option is rotation to a non-corn crop. 

Rotation will result in death of the entire rootworm population in the field because the larvae will not have a suitable host on which to feed and they will die. Since our rootworm beetles don’t lay many eggs in non-corn crops, the field can be planted the following year with no risk of a damaging rootworm population. 

When rotating to a non-corn crop, the volunteer corn that germinates must be killed when small to prevent rootworms from surviving and developing to beetles. The surviving beetles might lay eggs and re-infest the field, and the presence of corn in the field could attract other beetles from a considerable distance.

Of course crop rotation is often not an option, so here are the answers to some commonly asked questions. These answers are based on a field being in the resistance zone for Cry3-type toxins. If fields were planted to these toxins for the last several years and had lodging and high numbers of beetles, then resistance is likely. 

Is there any difference between a Cry34/35 (only) hybrid and one that has both Cry34/35 and a Cry3-type toxin?

It is better to plant corn with a pyramid of toxins rather than Cry34/35 alone. Resistance to the Cry3-type toxins is not complete so, in pyramids of the two types, the Cry3 will still provide some measure of additional root protection over Cry34/35 alone. This “partial protection” will also help preserve rootworm susceptibility to Cry34/35 because some of the insects with resistance alleles for Cry34/35 will be killed by Cry3-type toxins and won’t pass genes on to the next generation. See the table below for a full list of Bt corn hybrids active against corn rootworm, and the type of toxin(s) they contain. 

Several studies by academics and the seed industry have shown that, in areas where there is resistance to Cry3-type toxins, pyramids of Cry3s and Cry34/35 do not benefit from the addition of soil applied insecticides. Similarly, in our area there is probably no economic benefit from using soil applied insecticides on pyramids of Cry34/35 plus a Cry3-type toxin.

If I can use a Bt corn that has Cry34/35 without a Cry3-type toxin, will my roots be protected? 

Probably. There is no known resistance to Cry34/35 in our area and root protection should be very good. However, there are some caveats. One caveat is that, due to resistance to Cry3-type toxins, some fields have enormous numbers of eggs in them and the Cry34/35 will be challenged. We have seen instances of significant root damage in Cry34/35 corn under heavy rootworm pressure. If a continuous corn field had extremely high numbers of beetles last year and adult control was not used, then it might pay to use a soil applied insecticide when planting Cry34/35 seed. 

The other caveat is that toxin expression is lower in plants grown under stress, so proper agronomic conditions need to be met if the Cry34/35 is going to do the best job possible.

What if I have resistance but have to plant a Cry3-type (only) toxin?

In this case expect damage equal to or worse than last year, as a higher percentage of the population is now resistant. (Winter rootworm mortality is usually not a factor in our area, and adult sprays last year provided only suppression of egg laying.) The use of a high rate of insecticidal seed treatment and an at-plant soil applied insecticide is strongly recommended. 

How much protection is provided by insecticidal seed treatments, soil applied insecticides, and beetle sprays?

The answer varies by the amount of insecticide on the seed, but even at the highest amounts available on commercial corn seed, protection will be insufficient at moderate and higher infestations. If soil applied insecticides have been used continuously when planting Bt rootworm corn hybrids and spraying for beetles, the rootworms could also have developed resistance to the insecticides. 

Until this past year Bifenthrin insecticide has been used almost exclusively for both soil applications and beetle sprays. This has put a lot of pressure for selecting rootworm resistance. Researchers from Kansas State University and the University of Nebraska has shown different levels of resistance to Bifenthrin in rootworm populations across Kansas and Nebraska. Steward EC insecticide, which is not a pyrethroid, has recently received a supplemental label as a foliar application for beetle control. 

Does corn without a rootworm Bt toxin have a place?

Corn without a Bt toxin is a good choice for ground coming out of rotation to a non-corn crop because the rootworm pressure will be essentially zero. The seed will be treated with an insecticide and fungicide similar to the Bt hybrids. Planting non-Bt corn in a field that had high rootworm populations the previous year is not a good idea, even with high rates of insecticide in the seed treatment and with soil applied insecticides used at planting. 

Unfortunately, seed companies have not put as much breeding effort toward their non-Bt hybrids, so in some cases the agronomics and yield potential are inferior to the Bt hybrids. This is not true across the board, so consult your seed dealer(s) to examine the yield potential of hybrids that do not have rootworm protection. 

The Bt toxins in every type of hybrid from every seed company are listed in the Handy Bt Trait Table. 

Good News: Some Single Toxin Bt Corn Being Withdrawn from the Market

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.

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More background on how we got here can be found in previous editions of this newsletter:

Shuffling the Deck Chairs in Bt Crops (2016) 

Southern Bt Crops: Getting Boxed In (2017) 

Texas Panhandle Corn Rootworm Probably Resistant to Some Bt Corn (2018)

Virus-Based Insecticides for Control of Headworms in Sorghum and Caterpillars in Other Crops

A "new" insecticide option is being tried and promoted on the High Plains for control of some caterpillar pests, especially sorghum headworms (corn earworm and fall armyworm). There are some appealing aspects to this, but there is a lot we don't know as well.

Dr. Ed Bynum just posted a nice summary of the history of these insecticides based on nucleopolyhedrovirus (NPV), how they work, and some aspects of application and labeling. Insecticidal Virus Products for Pest Control: What are the Latest Facts, Panhandle Pest News, 7 November 2018.

Because these are live viruses and spread in the field, the minimum plot size needed for research is very large. We would be interested in trying one or both of these headworm products on large fields next year. By large I mean way bigger than we have at the Experiment Station. There would be large blocks of treated and untreated sorghum, and ground application equipment would be essential.