A research project is underway 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 in 1991-93 with some success for sweet corn producers in that region. T. ostriniae wasps aren’t used much 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%.

Also, 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. The  release of T. ostriniae in commercial dry bean fields against the western bean cutworms was studied  in 2016 for the first time. Below are the initial findings from this first season of data.

Study design
In 2016, T. ostriniae 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. ostriniae 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. Unfortunately, recovery on sticky cards was very low with only 0.01%  to 0.02% of the population recovered, and almost no parasitized eggs were recovered.

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, with a progressively declining recapture as sampling locations moved away from the release point.

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.

These functions will be important to determine in the future so that an effective number (but not an excessive amount) of T. ostriniae wasps can be released.

The 2017 plan will include a massive increase in the number of T. ostriniae 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.

Bradshaw is an associate professor of entomology and Extension specialist at UNL's Panhandle Research and Extension Center.