Crop Comments: Super-weeds, now super-bugs

My Canadian cyber-sentry E. Ann Clark, PhD… retired agronomy professor from Ontario’s Guelph University… e-mailed me an article titled “Former ‘Billion Dollar Bug’ is Mounting a Come-back”. The sub-title read: “The corn rootworm pest has developed resistance to GM (genetically modified) Bt insecticidal toxin.” This article, appearing June 15, 2018, online at AgWeb.com, was written by Sonja Begemann.
Begemann wrote that prior to Bt technologies farmers lost $1 billion annually to corn rootworm (CRW) — in the form of chemical costs or actual yield loss. With resistance to traits that once countered this insect’s rampant anti-corn insult, CRW, quoting Begemann, “Might just nibble its way back to a billion-dollar price tag.” CRW poses a double threat: first, the adult snips corn silks, a problem which, if unchecked, prevents successful pollination and kernel development; secondly, larvae munch on roots, which leads to risk for disease and plant stress. CRW was once controlled by GM traits, but with resistance threatening to run rampant, biotech plant scientists have been coerced back to the drawing board to try to slow the spread of CRW’s resistance to Bt. Corn industry think-tank enthusiasts have proposed the “refuge-in-a-bag” (RIB) method, whereby GM Bt seeds are mixed with non-GM Bt seeds, so as to delay (not prevent) resistance by maintaining a population of Bt-susceptible pests.
In a nut-shell, genes that govern the production of Bt toxins (in the original B. thuriengensis) are inserted into host corn plants, thus becoming part of the affected corn specimen’s genotype… to be passed onto the next generation. Without getting too complicated, the Bt-toxin producing genes (in both the original bacterium and the host corn plants) produce crystalline protein poisons, hence the “Cry” designations… such as Cry34/35Ab1 or eCry3.1Ab. Different Cry proteins (toxins) target different insects. According to the National Center for Biotechnology Information (www.ncbi.nlm.nih.gov): “Cry proteins are parasporal inclusion (Crystal) proteins from Bacillus thuringiensis that exhibit experimentally verifiable toxic effect to a target organism or have significant sequence similarity to a known Cry protein.” (Parasporal describes a crystalline protein that forms around a spore in some bacteria. This encapsulation product acts as a toxin precursor when digested.)
With biotechnology, not every game plan involves transgenics (moving one gene from species to a totally different one). For instance, researchers at University of Illinois have investigated the prospect of two naturally-occurring resistance genes in corn. According to Martin Bohn, corn breeder in the Department of Crop Sciences at U. of I., “One interacts with nematodes in the soil and tells them to attack the rootworm larvae. The other is related to the plant’s ascorbate synthesis pathway that produces free radicals that injure feeding insects. “We were screening [corn lines] for insect resistance. There were not many, but we found some. We had to look into lines from Argentina, Brazil and the Caribbean Islands to find it. (This was good because) our previous research showed that there is no inherent resistance in the elite hybrids grown by most farmers in the Midwest.”
Dalton Ludwick, post-doctoral researcher at Virginia Tech, working with USDA, commented on the CRW resistance issue further: “The first documented case (of CRW resistance) was in Iowa in 2011 to Cry3Bb1. Now we have documented issues in Iowa, Illinois, Minnesota, Nebraska and North Dakota — there are other areas it’s been talked about, but no formal documentation has been published. Farmers who practice corn-on-corn are at the greatest risk of developing resistance. In lab studies researchers proved it only takes up to four years of pressure to select for CRW resistance on three of the four (Cry) proteins on the market. When the pest develops resistance to a protein, the result is the Bt technology within a specific hybrid can be rendered ineffective or crippled. EPA, industry and others have created best management practices for CRW. Crop rotation is the best option… if you can’t rotate, plant something with two modes of action, if you can’t do that, use a single mode of action and a soil-applied insecticide — that’s the bare minimum. If you can’t do that, and have CRW present, you’re out of luck.”
Professor Clark… with back-up from the GM Watch website (www.gmwatch.org)… made some comments: “Refuge-in-a-bag” doesn’t work in delaying pest resistance to Bt maize. “This because, in order to prevent pest resistance to Bt toxins, maize (corn) farmers in the U.S. are required to plant “refuges” of non-Bt maize near the Bt fields. The idea is that the refuges will sustain populations of pests that are susceptible to Bt toxins. However, previous studies have shown that refuges do not work well, for three reasons: farmers don’t comply with refuge requirements, pests are able to live and reproduce in Bt maize fields, and the non-Bt refuge plants become contaminated by cross-pollination with Bt toxin-producing genes.”
Clark further explains that poor compliance by farmers in planting refuges has led to seed companies selling seed mixtures, known as “refuge-in-a-bag”, which contain a percentage of non-Bt seeds mixed in with the Bt seeds. Now a study part-funded by seedcorn companies and the USDA confirms that “refuge-in-a-bag” does not work because the non-Bt maize becomes contaminated with Bt toxin genes through cross-pollination. This joint study found that over 90 percent of maize kernels expressed at least one Bt (Cry) protein. In addition, the surviving pests on the refuge plants did not thrive, meaning that no viable Bt-susceptible pest populations survived. Which means the proportion of Bt-tolerance in the CRW gene pool continues growing at an unchecked rate.
My solution to the problem of CRW population becoming increasingly resistant to Bt toxins is fairly simple: plant sorghum the year before planting corn. Sorghum poses a prussic acid threat when first growth is grazed too short (less than 20 inches)… or an attempt at regrowth following killing frost is grazed too soon. Prussic acid is toxic to rootworms attacking sorghum… and the prussic acid is strong enough to carry over to next year and protect the corn crop.

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