Parasitoid wasps are increasingly used to combat crop pests as the most friendly method to the environment. These insects deposit their eggs inside the body of the larva or adult of another insect species. The wasp's larvae develop inside the body of the host consuming it and finally the host dies.

A team of entomologists from Montana State University are investigating ways of increasing the availability of the parasitoid wasps for the farmers. The team made a two years study that involved mass-rearing a species of parasitic wasps specialized in the wheat stem sawfly larvae that produce great damages in cereal crops. Large cages filled with wasps were placed in wheat fields deliberately infested with sawfly. The scientists wanted to see if supplemental food provided as nectar from flowering plants or as honey water increases the number of parasitic wasps produced in each cage. Also it was checked if the use of 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" said entomologist David Weaver.

Supplemental food seems to be more important at lower parasitoid densities, which is definitely true in natural situations. "Right now, the number of people wanting parasitic wasps far out-number what we can deliver," he said.

The small parasitic wasps are part of a very efficient natural suppression system of wheat stem sawfly that varies greatly from field to field. The wasps produce two generations of offspring a year, compared to only one for the wheat stem sawfly.

Thanks to a pilot scale research, many zones in Montana have now established populations of parasitic wasps in their wheat fields. Gaining the capacity to reliable wasp mass production would be a victory, because the process of introduction is inefficient.

Small increases in efficiency means thousands of additional parasitic wasps, which could infest new wheat fields. Researchers also want to determine the optimal time to infest the fields with wasps.

The new study showed that enough parasitoids to establish a founding population in a sawfly infested wheat field exist in the straw residue from a single mass-rearing cage.

This kind of work is still more necessary because drought in the last years favored the spreading of the wheat stem sawfly in many areas in the US. "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.

"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."