The western bean cutworm is a serious pest of dry edible beans and corn in Nebraska, and is now expanding its range eastward to as far as Pennsylvania and Ontario.

Typically, farmers use chemicals to control western bean cutworms in dry beans; however, down the road there may be another option. A new biological control method that was tested in the Panhandle showed promise as an alternative control measure. UNL Extension Entomologist Jeff Bradshaw said more research is needed, but a 2011 study near Scottsbluff showed that the parasitoid, Trichogramma ostrinae, preyed on western bean cutworm eggs in dry beans and corn.

Trichogramma have been used successfully in China against the Asian corn borer and in the eastern United States against the European corn borer, so Bradshaw is encouraged by the fact that they also will prey on western bean cutworm in bean and corn fields.

Trichogramma ostrinae, a tiny wasp, is a native parasitoid of the Asian corn borer. In the early 1990s it was introduced into the eastern United States for control of the European corn borer in sweet corn. The Scottsbluff study was intended to find out whether trichogramma would prey on the eggs of western bean cutworm in corn and dry beans.

Bradshaw noted that much is not known about western bean cutworms. There are no highly accurate sampling programs in dry beans, and little knowledge about what factors control the spread of the pest.

Adding biological control to the arsenal of management options has several advantages, Bradshaw said. Chemical control has economic and environmental costs and requires aerial application due to the point in the season when it would be applied — when the rows have closed and equipment can not longer be driven through. The per-acre cost of aerial application is affordable only if there are enough acres to spray in one area. Researchers and some bean growers are experimenting with more upright, less viney plants that allow direct harvest with a combine, instead of a multiple-step process of cutting, windrowing and swathing. But most bean acres still consist of viney, low-growing plant types.

Trichogramma Targets Limited

Although the trichogramma wasp comes from China, it is not expected to become an invasive species in the United States, Bradshaw said.  Extensive studies from Cornell University and the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service (APHIS) have shown only 13 species of Lepidoptera, such as European corn borers, diamond back moths, and now western bean cutworms, are attacked.  It has also been used on the East Coast against European corn borers in sweet corn to economic benefit.

However, Mike Hoffmann, Ph.D., a researcher at Cornell University, found that a few species of moth, including the western bean cutworm, are susceptible in the laboratory and asked Bradshaw about testing the wasp’s effect on cutworms.

Trichogramma ostrinae lay eggs inside western bean cutworm eggs that have been deposited in sacs on field crops. Their larva develop inside and feed on the contents of the egg. If the host egg is large enough, it may even support the development of more than one trichogramma.

UNL Panhandle REC Adds Research for Dry Bean and Corn

The Scottsbluff project was carried out in 2011 by Fernanda Pelegrinotti, an intern from Brazil (UNESP, Universidade Estadual Paulista), under Bradshaw’s supervision. Cages were set up around corn and dry bean plots at two separate locations at the Panhandle Research and Extension Center at Scottsbluff. Four of the cages (two on corn and two on beans) were infested with both female western bean cutworm moths and parasitoids. Another two cages were infested with moths, but no parasitoids, as controls. After about 10 days of exposure, the western bean cutworm egg masses were evaluated.

Pelegrinotti found that the wasp will parasitize western bean cutworm, and in fact the parasitism rate was higher in dry beans than in corn. In addition, some of the cutworm eggs were victims of other mortality factors. The conclusion is that an augmentative release could significantly increase the mortality rate of western bean cutworm in dry beans, Bradshaw said.

Bradshaw said research conducted in eastern sweet corn fields on European corn borers showed that the parasitoids worked well as part of an integrated pest management (IPM) strategy in combination with chemicals.The overlaid insecticide did not kill off the trichogramma wasps.
Parasitized eggs also are noticeably darker than non-parasitized, so a field scout would be able to distinguish between the two and, in the future may be able to make a recommendation for treatment based on the rate of parasitism, he said.

Further Research Needed

The next step depends on money. So far, funding has not been secured to continue research, but there are more opportunities to present the project to funding agencies, he said.

Bradshaw said there are a lot of questions to answer: How does this treatment work in upright bean plants compared to viney? How large is the effective area of control — how far the parasitoids would spread from the release point? How long does it take for parasitoid eggs to emerge from cutworm eggs after they’ve been laid into the eggs, and how does that match up with the cutworm life cycle?