Crop rotations in corn back in spotlight

Called the “billion dollar beetle” in the U.S. Midwest, the western corn rootworm has been a huge economic issue for over 50 years. | Stefan Toepfer/CABI photo

New emphasis put on importance of rotations now that western corn rootworm has developed resistance to Bt varieties

The western corn rootworm has been a major pest in the United States for decades.

Called the “billion dollar beetle” in the U.S. Midwest, it has been a huge economic issue for over 50 years.

The pest is found across Canada. In 2016, it was first detected in British Columbia’s Fraser Valley with populations reaching record levels in local forage and sweet corn fields in 2017 in the Abbotsford and Chilliwack corn-growing areas.

In 2003, genetically modified Bt corn was introduced to produce proteins from the bacterium Bacillus thuringiensis that could kill rootworm. The technology allowed farmers to increase yields while, in some circumstances, reduce their dependence on insecticides.

But within six years the rootworm evolved resistance to Bt corn.

Yves Carrière, professor of entomology at the University of Arizona’s College of Agriculture and Life Sciences, said the first case of field-evolved practical resistance was detected in Iowa in 2009.

“The rootworm has now evolved resistance to both types of Bt toxins currently commercialized for its control, which are Cry3 (Cry3Bb, mCry3A, and eCry3.1Ab) and Cry34/35Ab toxins.”

To combat this resistance, crop rotations are strongly recommended.

Carrière is lead author of a study that evaluated the effectiveness of crop rotations in mitigating the damage caused by resistant corn rootworms.

Carrière and his team tested crop rotations by analyzing six years of field data from 25 crop reporting districts in Illinois, Iowa and Minnesota, all of which have faced some of the most severe rootworm damage to Bt corn fields.

By cycling different types of Bt corn and rotating corn with other crops, farmers greatly reduced rootworm damage. Most notably, crop rotations were effective even in areas of Illinois and Iowa where rootworm resistance to corn and soybean rotation had been previously reported.

“Our results suggest the greater value for mitigating corn rootworm problems in fields of Cry3Bb corn is rotating Bt corn with other crops such as soybean,” he said. “Rotating different types of Bt corn should be considered as a secondary tactic, especially given that some populations of the western corn rootworm have evolved resistance to both toxins.”

He added even if farmers do not have problems, they should still rotate corn with other crops as much as economically possible.

The data showed that across crop-reporting districts of the three U.S. states studied, a one percent increase in corn-soybean rotation was associated with a 3.7 percent decrease in the frequency of Cry3Bb corn fields with corn rootworm problems.

Crop rotations have several other benefits, including reduction in use of fertilizers, improvement of soil health, and control of other pests.

Another secondary tactic that can be used to delay resistance is to diversify the type of corn grown, but this was not as effective as rotating to different a type of crop.

Adult corn rootworms have one generation of larvae each year. They emerge in mid-summer and lay their eggs until early fall. Eggs diapause during the winter and the larvae emerge the following spring to feed on corn roots.

It was hypothesized that freezing in winter or coping with a rainy spring could reduce the survival of eggs and small larvae, respectively, and reduce the frequency of Bt corn fields being damaged.

Researchers found no evidence for effects of freezing and only weak evidence for negative effects of abundant rain in the spring. However, the team has unpublished data indicating that abundant spring rain does reduce some rootworm damage to Bt corn fields.

Strategies to manage the rootworm include the refuge strategy in which non-Bt crops are planted near Bt crops to allow survival of susceptible insects and delay their adaptation to genetically engineered crops.

The research was published in Proceedings of the National Academy of Sciences.

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