By Josh Lancette

A new study from researchers at Iowa State University found that using rye as a cover crop in cornfields can lead to increased populations of true armyworm (Mythimna unipuncta) in the corn, as well as increased defoliation.

Cover crops are typically considered to be beneficial. However, this study, which was published in the Journal of Economic Entomology, suggests that cover crops can actually do some harm by leading to an increase in insect damage. Preventing this negative effect of cover crops could lead to changes in farming practices.

In Iowa, much of the farmland is used for corn and soybean. During the off-season for these crops (i.e., winter), the soil is bare, which puts it at risk for erosion and other troubles. Some farmers plant crops in the off-season to help prevent soil erosion, prevent weeds, improve soil quality, and increase biodiversity in the field, among other advantages. These crops are called cover crops. Because rye grows during the winter months, it is often used as a cover crop for corn and soybean.

While cover crops have many positive effects, some farmers in Iowa began reporting insect injury to their corn in fields that had used rye cover crops. So, a team of researchers from Iowa State decided to investigate. Over the course of two years, they studied corn crops that had been preceded by a rye cover crop and corn crops that hadn’t been preceded by any cover crop.

“We found that although adult true armyworm were captured around all the cornfields we sampled, significantly more true armyworm larvae were found in the cornfields that followed a rye cover crop,” said Mike Dunbar, one of the researchers. “In cornfields without a rye cover crop, true armyworm larvae were only found in the rows bordering field margins, whereas larvae were found throughout cornfields that followed a rye cover crop.

Furthermore, incidence of defoliated corn plants was significantly greater when corn followed a rye cover crop, and in some cases injury exceeded economic thresholds. Defoliation occurred throughout cornfields that followed a rye cover crop, but was limited to rows bordering field margins in cornfields without a cover crop.”

This direct correlation between a rye cover crop and insect damage in cornfields due to the true armyworm is an important finding because it could influence farming practices.

For example, previous research suggests terminating rye cover crops by using herbicides or tillage at least two weeks before corn is planted.

“It’s generally thought that early termination [of a cover crop] will reduce the risk of insect injury by preventing a ‘green bridge’ for insects like armyworm to move from the cover crop to corn,” said Dunbar. “Many of the farmers we worked with used this approach. However, early termination of the cover crop was not always possible; spring weather can make field conditions too wet for farmers to enter fields to spray herbicide.”

Furthermore, even though this study revealed a negative side of cover crops, the researchers do not suggest abandoning them.

“Although we observed an increased risk of insect injury when corn follows a rye cover crop, we still believe that a rye cover crop can be beneficial to farmers, and we don’t think these results should prevent the use of a cover crop,” said Dunbar.

“The true armyworm can be managed, and there are recommendations that can help prevent pest outbreaks if the rye cover crop is not terminated appropriately. We hope our results will increase farmer awareness of the pests that may respond positively to cover crops, allowing farmers to realize the full benefit of a cover crop for improving soil health.”