Sunday, September 1, 2019

Lab Results Indicate Last Year's Rootworms Not Resistant to Bt Corn

Last year we had huge numbers of corn rootworm beetles in corn around Hart in early July, and I wrote at the time that they were "probably resistant" to the mCry3a rootworm toxin. I was seeing classic resistance signs: lodged corn in Bt fields, root damage, and many, many beetles. In that article I said that we had collected insects to send to a USDA lab where they would be assayed for resistance, and that I would report the results of the assay here in FOCUS on Entomology.

The results are in, but before I provide them I will tell a bit more of the story on how we arrived at these results. The 1,200 beetles we sent to the USDA-ARS Lab in Columbia, Missouri were healthy and laid many eggs. They were tested by USDA-ARS personnel, and personnel at the University of Missouri. Eggs were held over the winter at the USDA Lab, and when they were ready to hatch we asked the seed company that sold the corn, and the seed company that was the registrant of mCry3a for permission to assess the resistance level by using standard toxin overlay procedures on artificial diet. Both of these companies refused to allow the bioassay. Their justification was that the field where we collected the insects was not an official "Performance Inquiry", so they were not obligated under the terms of their EPA registration to test this population. (This is true, but why the resistance to finding out whether there was resistance?) However, Texas A&M has an agreement in place with the registrant of the toxin that we can use commercialized hybrids to assess resistance, so we went that direction.

Our Missouri colleagues sent the results last week, and they showed only a slight elevation in tolerance to mCry3a in the Texas population as compared to a known susceptible population provided by the USDA-ARS lab in Brookings, South Dakota. (The Brookings lab is key to maintaining several corn rootworm strains used in the investigation of resistance, as it is impossible to find non-selected corn rootworms in nature since Bt corn has been planted for so long over such a wide area.)

So, as promised, I have provided the results of the resistance screen for the insects we collected near Hart last year. I am indebted to our friends at the USDA-ARS lab in Missouri for doing this work. The official determination is that they are only slightly less susceptible to mCry3a than a population that is known to be susceptible. They are not, according to formal screening, resistant to mCry3a. I am still scratching my head as to why our mCry3a fields (and fields all the way to Colorado) had root damage, lodging and clouds of rootworm beetles last year, but I have to go with the science. And I completely trust my colleagues in Missouri.

In July of this year I visited the Hart area again and saw few beetles and little damage. In my opinion, the very heavy spring rains after planting drowned many of the small rootworm beetle larvae, and this in turn reduced potential damage, even to non-Bt corn. (This phenomenon is well documented in the Midwest, but not so documented here since we don't often have abundant spring rains.) Additionally, another contributor to the lack of high beetle numbers this year is the fact that growers abandoned, for the most part, hybrids that had only the mCry3a toxin and switched to hybrids that had the Cry34/35 toxin with or without mCry3a. This is good rootworm control and resistance management! The best option is to plant a hybrid that has Cry34/35 and any of the Cry3-type toxins. It is easy to determine which Bt toxin combinations are present in any particular hybrid by looking at the Handy Bt Trait Table.