On Aug. 24, 2012, in cooperation with Aaron Gassmann’s laboratory, Iowa State University, the evolution of field resistance by corn rootworms to the Cry3Bb1 protein was confirmed in some problem fields located in northwestern Illinois (Henry and Whiteside Counties). 

During the summer of 2012, Joe Spencer, Illinois Natural History Survey, received suspected Bt-resistant western corn rootworm adults that had been collected in continuous cornfields in McDonough, Mercer and Sangamon Counties. Eggs were obtained from these adults in the laboratory. Single plant bioassays utilizing larvae were performed from July through November 2013. The bioassay procedures were those described by Gassmann et al (2011).

The suspected Bt- (Cry3Bb1) resistant larvae were exposed to a hybrid expressing the Cry3Bb1 protein as well as its corresponding isoline (not expressing the Cry3Bb1 protein). Larvae obtained from three control colonies of western corn rootworms also were used in the bioassays. The control larvae had never been exposed to any corn rootworm Bt protein and were provided by the USDA North Central Agricultural Research Laboratory in Brookings, S.D. The results from these bioassays confirm the evolution of field resistance to the Cry3Bb1 protein in these problem fields located in McDonough, Mercer, and Sangamon Counties. This now brings the total number of Illinois counties with field-evolved resistance (Cry3Bb1 protein) to five.

Evolution Of Bt Resistance

Recently, Bruce Tabashnik of the University of Arizona's Department of Entomology and his co-authors published a paper in the Journal of Economic Entomology that provides definitions to 50 “key terms” regarding the evolution of resistance to Bt crops and pesticides.

There continues to be some controversy regarding the most appropriate procedures (plant-based vs. diet-based bioassays) that should be used to confirm whether or not resistance has developed by an insect population to a Bt protein. Tabashnik and his co-authors offer the following definitions that I believe are helpful in this on-going debate and useful in communicating with producers who have experienced greater than expected levels of damage to rootworm Bt hybrids in their fields.

  • “Resistance: genetically based decrease in susceptibility to a pesticide”
  • “Field-evolved resistance (= field-selected resistance): genetically based decrease in susceptibility to a pesticide in a population caused by exposure to the pesticide in the field”
  • “Practical resistance (= field resistance): field-evolved resistance that reduces pesticide efficacy and has practical consequences for pest control”

Tabashnik and his co-authors described five cases of practical resistance that evolved within five insect species — three to Bt corn, two to Bt cotton. One of the insect pests of corn described is the western corn rootworm. It has become increasingly evident that some producers have experienced a loss of efficacy with some Bt hybrids in their fields in recent years. To date, most of those fields have been in continuous corn production and producers have not rotated traits. It’s also clear that “practical consequences” have resulted due to the loss of efficacy associated with some Bt hybrids in problem fields. The primary consequence so far has been an escalation in the use of planting-time soil insecticides with Bt rootworm hybrids. This practice may hasten the onset of resistance evolution to Bt proteins as outlined in a paper published last year (Petzold-Maxwell, J.L., L.J. Meinke, M.E. Gray, R.E. Estes, and A.J. Gassmann. 2013. Effect of Bt maize and soil insecticides on yield, injury, and rootworm survival: implications for resistance management. Journal of Economic Entomology 106(5): 1941-1951).

Another consequence of the evolution of practical resistance by the western corn rootworm has been the increased use of pyramided Bt hybrids, the use of seed blends as a primary refuge strategy, and a reduction in the size of the refuge — from 20% to 5% for some products. The reduction in the refuge size remains troubling for many entomologists, especially in areas of the Corn Belt where resistance has been confirmed to one of the rootworm Bt proteins. Although a seed blend offers many advantages including ensured compliance as well as improving the chances that Bt susceptible and resistant adults will mate, increased selection pressure on the one effective Bt protein within a compromised pyramid, may be a primary consequence of a 5% refuge.

To date, four Bt rootworm proteins have been registered for use in the United States: Cry3Bb1, Cry34/35Ab1, mCry3A, and most recently eCry3.1Ab. Very recently, Aaron Gassmann and his colleagues published a paper in which they confirmed the field evolution of resistance to the mCry3A protein. In the same journal article, they also confirmed cross resistance between Cry3Bb1 and mCry3A proteins.

These findings are troubling from many perspectives. For instance, pyramided Bt hybrids that are used in areas of the Corn Belt where practical resistance to the Cry3Bb1 and mCry3A proteins is a reality — may in fact be relying to a great extent upon the efficacy of Cry34/35Ab1 or eCry3.1Ab to ensure adequate root protection — again, at reduced refuge size requirements (5%).

Best Management Practices

On March 5, 2014, the minutes (No. 2014-01, 72 pages) of a scientific advisory panel were published: “A Set of Scientific Issues Being Considered by the Environmental Protection Agency Regarding Scientific Uncertainties Associated with Corn Rootworm Resistance Monitoring for Bt Corn Plant Incorporated Protectants (PIPS).” On page 23 of the document, included within a panel summary, the panel indicated that it was “unrealistic to rely upon refuges alone to achieve IRM for WCR given current Bt hybrids.” They listed several factors to bolster this statement: 1) Lack of a high dose in rootworm Bt hybrids 2) Resistance alleles are not rare 3) Resistance is probably polygenic 4) Random mating is not occurring 5) Cross resistance may exist between Cry3Bb1 and mCry3A (we now have confirmation), and 6) Ability to detect incipient locations of resistance and manage them to limit their spread is not working because...

  • “Zero foot traffic following planting”
  • “Lack of spatial continuity in the occurrence of problem fields”
  • “Major disincentives for reporting damage”
  • “Measuring damage is difficult because it occurs underground”
  • “Method for measuring damage (NIS) is only poorly correlated to population abundance”
  • “Rescue treatments do not exist”
  • “Scouting is imprecise and limited”

Other segments of this US EPA document describe how the assumptions used within previous resistance management models, regarding Bt hybrids and corn rootworms, need to be modified. The panel recommended that future models should evaluate how increasing the implementation of best management practices (BMPs) would affect the evolution of Bt resistance by this insect pest.

While the greater implementation of best management practices is a step in the right direction — let’s be clear, these practices should have been in place when Bt corn rootworm hybrids were first used over 10 years ago. Accelerated reliance upon the pyramided Bt rootworm products with reduced seed blend refuges will not solve this resistance management challenge. Increased use of soil insecticides, along with Bt rootworm hybrids, will likely only exacerbate resistance development. 

Producers should implement a long-term, integrated pest management approach for corn rootworm, including the use of multiple tactics over time (not all in the same season), such as use of a more diverse crop rotation system, use of a non-Bt hybrid in conjunction with a planting-time soil insecticide, rotation of pyramided Bt hybrids, and consideration of an adult suppression program in heavily infested fields.

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