As corn earworms (Helicoverpa zea) develop resistance to corn engineered to be toxic to them, a new study shows adult moths that survive have wings that are longer and stiffer, which favors long-distance migration—potentially helping the pests spread resistance faster and undermining a key crop-protection strategy. (Image by allenbyran via iNaturalist, CC BY-NC 4.0)

By John P. Roche, Ph.D.

Corn earworm moths (Helicoverpa zea) are a serious problem pest of corn, cotton, and other crops in the southeastern U.S. Since the 1960s, pest managers have used a soil bacterium called Bacillus thuringiensis (Bt) for control because it produces proteins toxic to insects. And in the 1990s, scientists learned how to introduce Bt genes into corn, allowing Bt-modified corn to produce Bt biopesticide proteins.

But pests can evolve resistance to Bt proteins, and since the corn earworm can migrate long distances, alleles for resistance can spread rapidly over wide areas. In addition, some previous studies have found that exposure to Bt can cause changes in the shape of corn earworm wings, potentially enhancing their migration ability.

To build upon these studies, a group of researchers in Australia and the United States measured wing morphology in corn earworm moths exposed to different regiments of Bt corn to test for effects on the moths’ wing shape. Their findings were reported in November in Environmental Entomology.

In a study published in 2023, Daniela Pezzini, Ph.D., then a doctoral student at North Carolina State University (now a field entomologist at Bayer) and colleagues tested the effects of Bt corn on flight of corn earworm moths in a flight mill, a device that tethers an insect to an arm connected to a rotating axis. As a test insect flies around the central axis, aspects of its flight can be measured. In their study, they found that exposure to Bt corn did not affect flight characteristics, including flight distance, in H. zea.

Researchers collect corn samples in the field for a study on how Bt corn affects wing shape of corn earworm moths (Helicoverpa zea). (Photo courtesy of Dominic Reisig, Ph.D.)

In the new study, Katrina Mikac, Ph.D., professor at the University of Wollongong in Australia, and colleagues built upon Pezzini’s research using analysis of the shape of wings and measurements of how wing shape may affect wind-induced wing deformation that could influence flight capabilities. They examined the wing morphology of 143 H. zea individuals from field sites in North Carolina and South Carolina.

They measured the position of wing landmarks such as vein junctions to compare wing morphology from moths from the following four treatment groups:

• Treatment 1: control moths exposed to a pure stand of non-Bt corn,
• Treatment 2: moths exposed to a pure stand of Bt corn with two toxin proteins,
• Treatment 3: moths exposed to a pure stand of Bt corn with three toxin proteins,
• Treatment 5: moths exposed to corn from a seed blend of 80% Bt corn with three toxins and 20% non-Bt corn.

Moths in Treatment 1 were non-selected controls, moths in Treatments 2 experienced a moderate amount of selection from Bt exposure, moths in Treatment 3 experienced intense selection from their Bt exposure, and moths in Treatment 5 experienced a moderate amount of selection from Bt exposure. (Moths from an additional treatment group, Treatment 4, were discarded because the sample size was too small for analysis.)

“The beauty of working with the insect’s phenotype (and wing shape in particular) is that we see changes in size and shape within a few generations, something that you can’t detect as quickly (or at all sometimes) using the usual genetic markers ecologists and entomologists have at their immediate disposal,” Mikac says.

Mikac and colleagues tested the hypothesis that the wings of corn earworms that developed in pure stands of Bt corn (i.e., treatments 2 and 3) would not be different from moths that developed in seed-blend refuges of Bt and non-Bt corn (treatment 5) but would be different from those of control moths developing in pure stands of non-Bt corn (Treatment 1).

They found that in males, there was a significant difference in wing shape between Treatment 2 and Treatment 5, which contradicted their hypothesis. Males also showed a significant difference in wing shape between Treatment 2 and Treatment 3.

In females, they found a significant difference in wing shape between Treatment 3 and Treatment 5, in contradiction to their hypothesis. Females also showed a significant difference in wing shape between Treatment 1 versus Treatment 5.

Ears of corn collected for a study on how Bt corn affects wing shape of corn earworm moths (Helicoverpa zea). (Photo courtesy of Dominic Reisig, Ph.D.)

A student samples an ear of corn in the field for a study on how Bt corn affects wing shape of corn earworm moths (Helicoverpa zea). (Photo courtesy of Dominic Reisig, Ph.D.)

Comparisons of Wire Frame Wing Models

The investigators constructed wire frames corresponding to the key structural elements of the moth wings, such as vein junctions and where veins terminated, and compared those frames among treatment groups. They found that in females, wings from Treatment 5 (which were “moderately” selected), were longer and narrower, whereas the wings from Treatment 3 (intensely selected) and Treatment 1 (non-selected) were shorter and wider. Similarly, the wire frame analysis in males showed that males from Treatment 5 had wings that were wider and narrower than in Treatment 2. This and previous studies conclude that long, narrow wings are more effective and efficient for long-distance dispersal.

Measuring Elastic Deformation of Model Wings

Mikac and colleagues also made models of wings corresponding to the shapes of the moth wings in the study and then measured wing deformation (i.e., bending) when the models were exposed to wind. In these “elastic deformation measurements,” the investigators found that for both males and females, models of wings from Treatment 2 (moderately selected) showed the highest values of wing deformation (that is, the least stiffness) when exposed to wind.

Model wings of moths in Treatment 1 (non-selected moths) displayed the second highest magnitude of shape change in both males and females. Model wings of moths in Treatment 3 and Treatment 5 showed significantly lower wing deformation, with models from Treatment 5 showing the lowest.

High levels of wing deformation, as was seen in Treatment 2, may make wings less efficient for flight, while long, narrow, stiffer wings may be most effective for long distance dispersal. Thus, the use of Bt corn can potentially increase migration ability of moths, spreading resistance genes geographically and reducing the efficacy of Bt.

“We saw wing shape being affected by eating toxic Bt corn in a single generation,” says Dominic Reisig, Ph.D., professor and extension specialist at NC State and senior author on the new study, “and some of the moths developed wings that were better for long-distance migration.”

Because long-distance dispersal permits H. zea to spread Bt-resistant alleles, this study suggests that moderately selected moths exposed to corn from a seed blend of 80% Bt corn with three toxins and 20% non-Bt corn may be selected for longer wings that are more effective for migration. An implication of this finding is that the use of Bt corn in certain regimens may increase the spread of Bt resistant genes, lowering the efficacy of this important control method. Potential solutions may include using Bt regimens that are either non-selected (such as Treatment 1) or intensely selected (such as Treatment 3), rather than regimens that are moderately selected (such as Treatment 5).

“A lot of studies on flight are done on a flight mill. In previous studies using flight mills in my lab, we found no differences in flight when these insects were fed Bt versus no Bt,” Reisig says. “But using the wing-shape studies with Dr. Mikac, we were able to see differences. So, multiple modes of investigation may be needed to uncover the answer to a question.”

Mikac expanded on this point: “Multidisciplinary research teams, members each of whom bring a different set of skills and toolsets for analysis, mean that we can think about the problem before us in a different way.”

A more complete understanding of the dynamics of Bt-corn, the evolution of resistance, effects on wing shape, and the effects of long-distance migration will depend on continuing research. “We are just beginning to understand the impacts of local and migrant corn earworm on Bt crops,” Reisig says. “Recent studies from my program showed that local selection on Bt is important for resistance. However, a recent study from the program of Dr. [Silvana] Paula-Moraes also showed that many of the corn earworms in the system mid- to late-season are migrants. Understanding where these insects are coming from and what Bt crops they are selected on will be important in the future.”

John P. Roche, Ph.D., is an author, biologist, and science writer with a Ph.D. and postdoctoral fellowship in the biological sciences and a dedication to creating compelling narratives for readers. He authors books and writes materials for universities, scientific societies, and publishers. Professional experience includes serving as a scientist and scientific writer at Indiana University, Boston College, and the UMass Chan Medical School; as a visiting professor at four tier-one schools; and as editor of science periodicals at Indiana University and Boston College.