There were a crazy lot of western corn rootworm beetles emerging from continuous corn fields this summer in the Texas Panhandle, and with seed purchase decisions on the horizon it might be a good idea to talk about Bt resistance.
For two years now, Dr. Ed Bynum, Extension Entomologist in Amarillo, and I have seen extensive damage to Bt corn with the toxin mCry3a. These fields have been found in the zone from Dalhart in the north to Hart in the south. We have been seeing all the textbook symptoms of a failed Bt rootworm toxin: growers using soil applied insecticides in addition to their Bt seed, heavy root damage, plant lodging, lots of adult beetles on the wing because they were not killed as larvae feeding on roots, and aerial spraying of adult beetles to try and kill them before they clip silks or lay eggs that will become rootworm larvae the following year.
This year we received phone calls from consultants around Hart that were alarmed at the huge numbers of beetles and lodged plants in mCry3a corn. Dr. Bynum, Dr. Katelyn Kesheimer and John David Gonzales, IPM Agents in Lubbock/Crosby and Parmer, Bailey and Lamb counties, respectively, and I dug corn roots from several continuous corn fields near Hart. The roots from Cry34/35 corn were fine; only slight damage was observed. Roots from mCry3a corn rated 0.80 to 1.6 on the Iowa State rootworm injury scale. (Yes, we used gene-check strips to make sure the fields were mCry3a.) In practical terms this means the rootworms had removed 0.8 to 1.6 nodes of roots. EPA and the seed companies have negotiated levels of damage that might reasonably indicate there is a resistance problem, and for single toxin Bt corn like mCry3a the level is 1.0 on the Iowa Scale. Single toxin Bt fields with root ratings above 1.0 are supposed to trigger collection of beetles for resistance screening.
Drs. Bynum, Kesheimer and Suhas Vyavhare (Extension Cotton Entomologist), and John David Gonzales and I collected 1,200 beetles from the mCry3a field mentioned above and sent them to a USDA-ARS lab for resistance screening. This process takes almost a year to complete and we will report the results when they come in. However, what we are seeing in mCry3a fields is highly consistent with what we would expect to see with resistance, so I am going to call it "probable resistance".
There are four Bt rootworm toxins in corn; mCry3a, eCry3.1Ab, Cry3Bb1, and Cry34/35. However, there is known cross resistance between the first three toxins listed above, so rootworm beetles experience these more like two toxins (mCry3a, eCry3.1Ab, Cry3Bb1) and Cry34/35. All four of these toxins have documented resistance, but resistance to the Cry3s is widespread, while resistance to Cry34/35 is only known in localized pockets – for now.
Corn rootworm has been shown to have the ability to become resistant to single toxins in as few as four seasons when the same toxin is used in consecutive years. Most of the continuous corn fields in the Panhandle have been in Bt corn for far longer than four years, and most of them have been planted to the same toxin(s) in hybrids from the same seed company.
The following table lists hybrids with only Cry3-type toxins for rootworm control. The toxins for caterpillar control, if any, are irrelevant to corn rootworm and are not listed.
By far the best way to manage corn rootworms in Texas is to rotate to a non-corn crop; the rootworm larvae will hatch next year and starve because they don’t have an acceptable host. Beetles will not lay an appreciable number of eggs in a non-corn crop, so the field can be planted back to corn after just one year out of corn.
However, crop rotation is not economically feasible in many circumstances, nor is planting a corn hybrid that does not have Bt for corn rootworm control. With pending seed purchase decisions in mind, in cases of rootworm resistance to a single toxin it is important to rotate to an entirely different toxin the following season. (Remember that mCry3a, eCry3a.1Ab and Cry3Bb1 are not very different as far as corn rootworm is concerned.) As far as we know, there is no resistance in Texas to Cry34/35. Every transgenic seed company has hybrids with Cry34/35, sometimes as the sole rootworm toxin but often in combination with one or more of the three Cry3-type toxins mentioned above. It might be a good time to contact your seed dealer and determine whether there are agronomically acceptable hybrids with Cry34/35 that can replace the hybrids that have only Cry3-type toxins.
Determining the types of Bt toxins present in hybrids from every seed company is easy to do, just visit the Handy Bt Trait table.