Growing pest resistance shows GM Bt crop technology not sustainable

Corn earworm pest evolves resistance to multiple Bt insecticidal toxins

The corn earworm pest is showing increasing resistance to the insecticidal toxins engineered into GM Bt corn, according to a new study published in PLOS One.

The authors state that “Many sweet corn farmers in Maryland either have stopped growing” GM Bt corn or “are applying more insecticide sprays to compensate for the reduced control efficacy”.

They also note that the resistance is likely to increase and spread in future due to a number of factors, including
* decreased farmer compliance with refuge recommendations, where an area is planted with non-Bt crops to try to delay Bt resistance building up in pests
* the high adoption rate of Bt crops, which exposes large numbers of pests to the Bt toxin continuously, thus increasing selection pressure for resistant pests;
* low expression of the Bt toxin in these crops, meaning that pests are not killed by the toxin but live to evolve resistance to it; and
* cross-resistance, where exposure to one type of Bt toxin leads to resistance to other types of Bt toxin, meaning that GMO industry attempts to defeat pests by stacking different types of Bt toxin in one crop are doomed to fail.

The authors conclude, “Based on field efficacy comparisons and lab bioassays, we demonstrate that corn earworm has evolved resistance to multiple Cry [Bt insecticidal] proteins expressed in genetically engineered corn.”

This paper adds to the large and growing body of evidence showing that the GMO built-in-insecticide model of pest resistance is ineffective and unsustainable. It is time to stop wasting valuable time and resources on this failed technology and rapidly move to proven effective agroecological pest management strategies involving diverse cropping systems, rotations, and biological controls.

Below are the authors’ summary of the new paper and the study abstract and conclusion.

Photo: Sarah Zukoff, Statesboro GA, USA

Authors’ summary

What key achievement does this paper describe?

Crops engineered with genes from the bacterium Bacillus thuringiensis (Bt) to express specific proteins (Cry proteins) are a major tool to manage agricultural insect pests. In 2015, 81% of all corn planted was engineered genetically with Bt for controlling insects (moths and beetles). Biotechnology enables the engineering of crops to express one or more (called pyramided traits) Bt Cry proteins simultaneously. However, with widespread use of genetically engineered crops, the targeted insects developing resistance to these proteins is a major threat to the sustainability of the Bt technology. Resistance development is particularly a concern for some of the pest insects for which the expressed Cry proteins are not highly toxic, enabling a few to survive. In this article, we describe the field-evolved resistance in corn earworm, a major pest of field crops, to multiple Cry proteins expressed by genetically engineered sweet corn.

Why would the public be interested in this result?

Broadly, the use of Bt crops has helped reduce insecticide usage in the U.S. However, many sweet corn farmers in Maryland either have stopped growing the Bt sweet corn hybrids described in our study or are applying more insecticide sprays to compensate for the reduced control efficacy. The Cry proteins expressed by genetically engineered sweet corn, to which corn earworm is now resistant in Maryland, are the same used in field corn for controlling lepidopteran pests (moths). Increasing damages by corn earworm to Bt field corn are reported already in North Carolina and Georgia, and corn earworm could become a major pest. We predict that corn earworm resistance to the Cry toxins is likely to increase, and spread. Therefore, the risk of damage to corn production across a large portion of U.S. is high. Our results, thereby have important implications for sustainable corn production, biotechnology regulatory policies, and sustainability of the Bt biotechnology.

How does this result improve upon/advance what has been done before?

The key finding from our study is the resistance development to pyramided Bt traits. Previously, resistance development to Bt crops worldwide are reported in 5 insect species (including corn earworm), but they all are against single Cry protein expressing crop varieties. Also, while corn earworm resistance to Cry protein (Cry1Ac) in cotton is already known, this is the first report of corn earworm resistance to Cry proteins expressed in corn.

How does this result change our understanding of nature, technology and/or the scientific process?

U.S. Environmental Protection Agency, as part of registration of this bio-pesticide, requires biotech companies to monitor for possible resistance development in corn earworm populations; however, to date sampling insects for testing have focused on the southern production regions where both Bt corn and cotton are grown. With Maryland populations already resistant, this study demonstrates the need for changes in some of the insect resistance management (IRM) and monitoring strategies.

Non-Bt crops grown in proximity or amidst Bt crops as refuges are part of IRM strategies. This strategy allows survival and mating of susceptible insects with Bt resistant insects, thereby delaying resistance. For corn hybrids expressing multiple pyramided Bt proteins for control moth pests, EPA approved the refuge-in-the-bag (RIB) strategy with reduced refuge requirement (95% Bt : 5% Non-Bt) in the corn belt. Based on our study results, the RIB strategy might not sufficiently delay resistance. Our results also emphasize the need for resistance monitoring for all Cry proteins registered against corn earworm, in northern regions of corn production including the corn belt. Also, sweet corn as a sentinel host crop is a better diagnostic tool for monitoring early stages of resistance.

How will this result affect future research directions, for yourself and others?

The mechanism through which corn earworm developed resistance to the pyramided traits needs investigation. We are currently examining the interactive influence of increased Bt adoption and climate change (temperature anomaly), on resistance evolution in corn earworm to Cry1Ab expressing Bt corn. This is relevant given the predicted increase of overwintering range and climatic suitability for many of target pests.


We started monitoring field efficacy and damage susceptibility of Bt corn variety since its commercial availability in 1996. Two decades of field experiments, comparing non-Bt and Bt (Cry1Ab expressing sweet corn) side by side in Maryland show a significant increase in ear damage by corn earworm across years. In addition to this single Cry protein expressing sweet corn variety, field experiments since 2010 also show corn earworm resistant to sweet corn expressing two Bt Cry proteins (pyramided variety expressing Cry1A.105+Cry2Ab2). Larvae successfully damaged an increasing proportion of ears, consumed more kernel area, and reached later developmental stages (4th - 6th instars) in both types of Bt hybrids (single and pyramid) since their commercial introduction. These damages were not due to changes in population densities of corn earworm, which declined over past two decades as observed from light traps. Also, there was no difference among the commercially available Bt varieties expressing Cry1Ab, so the increasing damage was not due to a variety effect.

Further, we established a resistant Maryland strain as a lab colony and compared it with a susceptible corn earworm stain. When we fed both strains different concentrations of Cry1Ab from corn leaf tissue, the Maryland strain gained more weight at all concentrations tested than the susceptible strain indicating resistance development. Based on field efficacy comparisons and lab bioassays, we demonstrate that corn earworm has evolved resistance to multiple Cry proteins expressed in genetically engineered corn.

Field-Evolved Resistance in Corn Earworm to Cry Proteins Expressed by Transgenic Sweet Corn

Galen P. Dively, P. Dilip Venugopal, Chad Finkenbinder
Published: December 30, 2016



Transgenic corn engineered with genes expressing insecticidal toxins from the bacterium Bacillus thuringiensis (Berliner) (Bt) are now a major tool in insect pest management. With its widespread use, insect resistance is a major threat to the sustainability of the Bt transgenic technology. For all Bt corn expressing Cry toxins, the high dose requirement for resistance management is not achieved for corn earworm, Helicoverpa zea (Boddie), which is more tolerant to the Bt toxins.

Methodology/Major Findings

We present field monitoring data using Cry1Ab (1996–2016) and Cry1A.105+Cry2Ab2 (2010–2016) expressing sweet corn hybrids as in-field screens to measure changes in field efficacy and Cry toxin susceptibility to H. zea. Larvae successfully damaged an increasing proportion of ears, consumed more kernel area, and reached later developmental stages (4th - 6th instars) in both types of Bt hybrids (Cry1Ab—event Bt11, and Cry1A.105+Cry2Ab2—event MON89034) since their commercial introduction. Yearly patterns of H. zea population abundance were unrelated to reductions in control efficacy. There was no evidence of field efficacy or tissue toxicity differences among different Cry1Ab hybrids that could contribute to the decline in control efficacy. Supportive data from laboratory bioassays demonstrate significant differences in weight gain and fitness characteristics between the Maryland H. zea strain and a susceptible strain. In bioassays with Cry1Ab expressing green leaf tissue, Maryland H. zea strain gained more weight than the susceptible strain at all concentrations tested. Fitness of the Maryland H. zea strain was significantly lower than that of the susceptible strain as indicated by lower hatch rate, longer time to adult eclosion, lower pupal weight, and reduced survival to adulthood.


After ruling out possible contributing factors, the rapid change in field efficacy in recent years and decreased susceptibility of H. zea to Bt sweet corn provide strong evidence of field-evolved resistance in H. zea populations to multiple Cry toxins. The high adoption rate of Bt field corn and cotton, along with the moderate dose expression of Cry1Ab and related Cry toxins in these crops, and decreasing refuge compliance probably contributed to the evolution of resistance. Our results have important implications for resistance monitoring, refuge requirements and other regulatory policies, cross-resistance issues, and the sustainability of the pyramided Bt technology.

Published: 31 December 2016