Rearing an army to save wheat

With wheat stem sawfly natural enemies in demand, Montana State University entomologists are investigating ways of increasing their availability.

This fall, the entomologists are concluding a two-year study that involved mass-rearing parasitic wasps that attack wheat stem sawfly larvae that tunnel the interior of developing wheat plants. The team includes entomologists David Weaver, master's graduate Godshen Pallipparambil-Robert and undergraduate Melissa Frazier of Kalispell.

Pallipparambil-Robert's work, as part of his completed master's degree, used large cages placed over wheat at the Post Agronomy Farm west of Bozeman, Mont. He deliberately infested the enclosed wheat with wheat stem sawflies, and then introduced the parasitic wasps. His research explored whether supplemental food provided as nectar from flowering plants or as honey water increases the number of parasitic wasps produced in each cage. Another part of his thesis project examined whether using special ultraviolet and visible light-transmitting windows increases the number of parasites.

"After two years, the research shows that the added light consistently causes small increases in the number of parasitic wasps, while the food supply is probably not important in these mass-rearing cages, because the parasitoids were added in large numbers, and attacked the available sawflies before the need to feed may have become critical " Weaver said. At lower parasitoid densities, supplemental food might be much more important, and research from other systems suggests that this is definitely true in natural settings. However, the goal of the research is to find ways to increase the supply of parasitoids from a controlled system to Montana wheat growers.

"Right now, the number of people wanting parasitic wasps far out-number what we can deliver," he said. The small, orange parasitic wasps are part of the naturally occurring suppression of wheat stem sawfly that varies greatly from field to field throughout Montana. The wasps produce two generations of offspring a year, compared to only one for the wheat stem sawfly.

A number of Montana counties now have established populations of these natural enemies in their wheat fields, thanks to pilot scale research co-sponsored by the Montana Wheat and Barley Committee; USDA, CSREES Special Research Grants; the Montana Board of Research and Commercialization Technology; and the BNSF Railway Foundation. However, the process of locating parasitic wasp populations for redistribution is inefficient, and having the ability to reliably mass produce these organisms would be an asset.

Even a slight increase in efficiency could translate into the ability to produce thousands of additional parasitic wasps, which could then be distributed to wheat stem sawfly infested sites.

Pallipparambil-Robert has just begun work on a doctorate in entomology at the University of Arkansas. The late summer and autumn efforts are being completed by Frazier. It became her job to tend the cages of parasitic wasps every few days, after Palliparambil-Robert departed.

Weaver said more research is needed to determine precisely how many sawflies and parasitic wasps need to be added to each cage and to determine the best time to add the wasps. The current research shows that enough parasitoids can be produced to establish a founding population in an infested wheat field using the straw residue from a single mass-rearing cage.

Mass-rearing of the parasitic wasps is only one of three or four approaches for sawfly management that are being pursued at MSU. The drought conditions of recent years have made the work more pressing, since drought favors damage by wheat stem sawflies, which are now more widely distributed in Montana than years ago.

"There are larger areas of sawfly damage all along the Golden Triangle, the northern tier of Montana counties as well as the area around Lewistown, Circle and Jordan," Weaver said. Currently, MSU is specifically partnering with MSU Extension county agents to redistribute and monitor populations of parasitic wasps, as part of an expanded effort to establish the more effective parasitoid populations in those areas.

"We hope to see the populations there grow, and what Pallipparambil-Robert's work will do is to help us have the ability to have parasitoids more readily available for future efforts," Weaver said. "Right now, we have to locate a large population of these beneficial insects before we can redistribute them. If we can reliably have them available at a known location, we could do our job much more efficiently." Source : Montana State University