Panhandle Perspectives - Dec. 20, 2016

Parasitoid wasps studied to control western bean cutworm in dry edible beans
Trichogramma ostrinae sampling results graphs
Left: Cumulative number of adult Trichogramma ostrinae wasps at the two bean fields. Right: Average number of adult wasps sampled by sticky trap over six sample locations from 50 to 250 meters from release point on two dry bean fields.

Panhandle Perspectives
Jeff Bradshaw

By Jeff Bradshaw, Associate Professor of Entomology and Extension Specialist

Panhandle Research and Extension Center

A research project is under way at the Panhandle Research and Extension Center to help determine whether a parasitoid wasp can help control the western bean cutworm, a pest that causes serious damage to dry edible beans in the Nebraska Panhandle. 

Initial data from the first year, 2016, raised several questions, and more studies are planned for 2017.

The wasp, Trichogramma ostriniae, is an important native parasitoid of the Asian corn borer. In the early 1990s, efforts were made to establish it in the northeastern United States for control of European corn borer, an important pest of corn.

In New York several million female wasps were released from 1991-93 with some success for sweet corn producers in that region. Trichogramma wasps are still not much used for biocontrol outside of Asia, and information about its biology is limited. However, recent studies have shown that a single release (about 70,000 per hectare or 28,328 per acre) could provide season-long suppression of the corn borer and reduce ear damage by 50 percent.

Additionally, previous studies in New York found that the wasp persisted in commercial sweet corn fields after insecticide applications, making this parasitoid an ideal tool for integrated pest management. Therefore, we began to study the release of T. ostrinae in commercial dry bean fields against the western bean cutworms in 2016 for the first time. Below is the initial findings from this first season of data.

Study design

In 2016, Trichogramma ostrinae wasps were released onto two commercial dry edible bean fields in Scotts Bluff County on July 14 (“Hill” field) and July 15 (“Water Tower” field). Within each field, an area of about 40 acres was selected and the release point was centered in each field to serve as the experimental unit. Each experimental unit was then partitioned into a grid of 36 sampling units. About 2 million T. ostrinae wasps were released per field in the center of the experimental unit.

At the same time as the wasps were released, 36 newly-laid, western bean cutworm eggs were placed at the center of each sampling unit onto the underside of a bean leaf to serve as sentinel hosts. A sticky card was also placed at each sentinel host site. The sentinel egg masses and the sticky cards were collected and replaced with new egg masses and cards four times in July and brought back to the lab, where they were incubated.

 

Results

The Trichograma wasps were found to increase in number throughout the duration of the study via sticky card sampling (first graph). Unfortunately, recovery on sticky cards was very low with only 0.01 to 0.02 percent of the population recovered, and almost no parasitized eggs were recovered.

Trichogramma wasp close-up

Trichogramma wasps are incredibly small, only a few millimeters in length. The small, black flecks in this image are Trichogramma wasps. For perspective, the card which is covered with wasps is 20 mm across, or slightly more than ¾ of an inch.

The low recovery rate on sticky traps may indicate that not enough wasps were released to enable effective parasitism. Because of the low recapture rate, no clear distance by day function could be derived. However, there was some indication of an average-distance-by-release-point function (second graph), with a progressively declining recapture as sampling locations moved away from the release point.

Further research should help us quantify these functions in the future, so we can give advice on how many Trichograma wasps should be released to be effective, without being excessive.

The 2017 plan will include a massive increase in the number of T. ostrinae wasps released, with the intention of releases in the Nebraska Panhandle as well as in central and eastern Nebraska. A similar intensive sampling designed is planned for 2017 in Scotts Bluff County.