Conventional Soil Management Draws European Corn Borer Toward Bt Corn
By John P. Roche, Ph.D.
The European corn borer (Ostrinia nubilalis) is a non-native moth pest of corn and other crops in North America, Europe, and North Africa. Its larvae bore into corn plants, causing damage to stems, ears, and leaves. The corn borer is part of a complex system involving the moth, its corn plant host, and the soil in which the plant grows. Over the past twenty years, corn modified with a gene to express Bacillus thuringiensis (Bt) proteins that kill corn borers has been widely introduced in the U.S.; this Bt corn has dramatically reduced European corn borer populations in North America, saving farmers billions of dollars. Rebecca Schmidt-Jeffris, Ph.D., of the U.S. Department of Agriculture-Agricultural Research Service, Brian Nault, Ph.D., of Cornell University, and colleagues studied the system of interactions among moth pests, corn plants, and soil and found an intriguing interaction between conventionally managed soil and the European corn borer.
Soil is a critical component of this moth pest/corn plant/soil system. Healthy soils make plants healthier, which in turn makes plants more resistant to pests. Minerals are an important component of soil health: if their content is too high or too low, plant health is affected.
Previous research has found support for the hypothesis that organic fertilization prevents dramatic changes in soil mineral content, which makes crop plants grown on organic soil healthier and thus better able to resist herbivores.
Previous work by Larry Phelan and colleagues found that European corn borers laid more eggs on corn grown in conventionally managed soil than on corn grown in organic soil. Organically managed soil is thought to offer more mineral buffering, leading to fewer big changes in mineral content, which in turn leads to healthier plants. Phelan and collagues’ work found that organic corn had higher rates of photosynthesis and higher levels of protein, patterns seen by multiple investigators.
Schmidt-Jeffris and her colleagues wanted to test whether the European corn borer shows a preference for corn plants raised on conventional soil, which would provide what they call a potential “unintentional attract-and-kill” mechanism serving to further reduce European corn borer populations on conventional corn fields because of the presence of Bt corn there. Their findings are reported in a new article published December 30 in the ESA journal Environmental Entomology.
Their hypothesis was that European corn borers would prefer conventionally raised corn over organically raised corn for laying eggs in lab choice tests, and would be more abundant on conventionally managed corn fields than on organic corn fields. In lab tests of this hypothesis, they presented corn borers with corn leaves or whole corn plants grown on conventional soil and corn leaves or whole corn plants grown on organic soil. They then determined whether the moths laid more eggs on conventionally or organically raised corn. In field tests, they collected moths in pairs of fields: one conventionally managed field paired with one organic field.
To examine potential effects of mineral balance on corn plants (and on European corn borer preferences), they also measured the mineral content of the study fields, and of plants grown in organic versus conventional soil. Minerals measured included calcium, potassium, magnesium, sodium, phosphorus, and zinc. For soil, they also calculated mineral ratios for calcium:magnesium, magnesium:potassium, potassium:sodium, and phosphorous:zinc. They studied two types of corn plants: those that were engineered to produce Bt, and those that did not produce Bt.
In their soil measurements, the investigators found that organic soil had a higher magnesium:potassium ratio, a lower potassium:sodium ratio, and a higher pH than conventional soil. Plants grown in organic soil had more magnesium and more calcium than plants grown in conventional soil. They also grew taller and had more chlorophyll.
In their lab choice tests using pieces of corn leaves, Schmidt-Jeffris et al. found no significant difference in egg laying between organically raised or conventionally raised corn. But in the tests using whole corn plants, they found that the moths showed a significant preference for corn plants grown in conventional soil over those grown in organic soil.
European corn borer moths occur in two strains that differ in the genes coding for their female pheromones—a Z strain and an E strain. European corn borer populations in nature are a mixture of these two strains, with the Z strain being more prevalent in the midwestern U.S. and the E strain being more abundant in the eastern U.S. and Europe. In their moth-trapping tests in the field, the investigators found significantly more E-strain moths in conventional fields than in organic fields, but found no significant difference for Z-strain moths.
Therefore, this study found that European corn borer moths preferred to lay eggs on whole corn plants raised in conventional soil, and that for the E-strain moths, the corn borers were more abundant on conventional fields. Both findings support the hypothesis that the corn borers preferred corn plants grown on conventional soil, which is aligned with the possibility of the “unintentional attract-and-kill” mechanism of control that Schmidt-Jeffris and colleagues proposed. Mineral tests of corn plants in the study found that corn grown on conventional soil had significantly less magnesium and calcium than organic corn. The investigators suggested that European corn borers may prefer laying eggs on corn plants with lower magnesium and calcium concentrations.
In summarizing her findings, Schmidt-Jeffris says, “Soil health and fertility management interact with pest management and can do so in very unexpected ways. In our case, an herbivore attracted to corn grown in conventionally managed soil is demonstrating preference for a crop that is more likely to be toxic to it, due to the use of Bt-containing corn varieties.”
When asked about potential future research directions, Schmidt-Jeffris says, “It would be interesting to see if other lepidopteran pests controlled by Bt corn also have the same preference for plants grown in conventional soil. It would also be interesting to see additional studies looking at how soil health may impact herbivore populations on a larger scale, especially in crops where organic and conventional fields are in close proximity to each other.”
In choice tests with whole corn plants, the European corn borers preferred laying eggs on the conventionally grown corn, which had lower magnesium and calcium levels. Future studies could investigate whether magnesium and calcium levels are factors that contribute to the choices of the moths, or are simply correlated with other causal factors.
Corn modified with Bt proteins has greatly reduced populations of the European corn borer and saved agriculture billions of dollars. But the corn borer–corn plant interaction is not static. Insects are evolving resistance to Bt proteins, reducing Bt’s effectiveness, and global warming is projected to increase the range of the European corn borer. Both trends will create the need for new directions in research, and new strategies of control.
John P. Roche, Ph.D., is an author, biologist, and educator dedicated to making rigorous science clear and accessible. Director of Science View Productions and Adjunct Professor at the College of the Holy Cross, Dr. Roche has published over 200 articles and has written and taught extensively about science. For more information, visit https://authorjohnproche.com/.