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Reagan Colyer, MSU News Service
May 19, 2023

BOZEMAN – Research from a Montana State University alumna published recently in the journal Physiological Entomology could have tangible impact for Montana agricultural producers who deal with perennial damage from wheat stem sawflies. 

Laissa Cavallini, who completed her master’s degree in entomology in spring 2022, worked alongside professor David Weaver and department head Bob Peterson in the Department of Land Resources and Environmental Sciences in MSU’s College of Agriculture. The project examined two species of parasitic wasps that act as biocontrols for wheat stem sawfly. Cavallini explored the nutritional needs of those wasps to explore ways of boosting their effectiveness as biocontrols — a pest management tactic that involves using one organism to manage another.

The insects, called Bracon cephi and Bracon lissogaster, are small orange wasps that can detect the presence of wheat stem sawfly larvae inside a wheat stem. They then inject a paralyzing toxin into the sawfly larvae before laying their own eggs. When the wasp eggs hatch, the immature wasps kill and consume the immobilized sawfly.

B. lissogaster, a small wasp species that acts as a natural biocontrol to wheat stem sawfly, was the subject of a recent publication by MSU alumna Laissa Cavallini. Photo by Robert Peterson.

“Something interesting about these parasitoids and about wheat stem sawfly itself is that the organisms are all native,” said Cavallini, who completed her undergraduate work in her home country of Brazil before joining Weaver’s lab in 2018 as a graduate student. “What’s more, these two species are the only ones known to parasitize the wheat stem sawfly.”

That unique relationship means that B. cephi and B. lissogaster are naturally suited to act as biocontrols for wheat stem sawflies but are limited by a short lifespan in wheat fields. Cavallini’s work examined the nutritional needs of the parasitic wasps to see if their diet could increase their lifespan and potentially make them more effective management tools.

“I thought it was a nice opportunity to work with parasitoids and look into controlling insect pests in a way that’s less harmful to the environment,” said Cavallini. “We already knew that some parasitoids were able to feed on nectar, but we didn’t have a lot of information in the beginning. We saw an opportunity to see if that was the same here in Montana.”

Because Montana has a dry, arid climate, Cavallini said, it was necessary to identify whether the wasps could readily access plant nectar as a food and water source. Depending on the type of plant, a lack of water could mean the nectar forms crystals that are difficult to consume or, most often, the nectar is stored in a part of the plant that the small insects can’t easily reach. Cavallini built on research done by a previous graduate student, Dayane Reis, to determine whether ingesting sugar had an impact on the wasps’ lifespan. The insects were fed sucrose, the same type of sugar that they would get from plant nectar.

“We noticed that sugars helped them a lot,” Cavallini said. “They need this resource. Feeding on water, they would live for two to five days, and feeding on sugar, some of them lived for 60 days or longer.”

It was an important finding, Cavallini said, and it confirmed the hypothesis that nectar could make a large difference in the effectiveness of the parasitoids as biocontrols. But the team still had to gauge whether the wasps could access plant nectar on or near agricultural fields, so they next investigated whether the lab findings could be replicated in an agricultural setting and explored crops that could serve as a source of nectar.

Cowpea, a pulse crop that produces extrafloral nectar, could be a viable food source for two species of wasps that act as natural biocontrols for wheat stem sawflies.

Ultimately, the team identified cowpea as a potential partner crop to serve as a food source for the two parasitoid species. A type of pulse crop, cowpea was appealing for several reasons. It produces extrafloral nectar, meaning its nectar is more easily accessible for insects like B. cephi and B. lissogaster, providing an ideal food source to help them live longer and work more effectively in wheat stem sawfly management. Additionally, heat and drought tolerant cowpea also provides many of the same benefits as other pulse crops, like peas and lentils. It fixes nitrogen in soil, reducing the need for nitrogen fertilizer, and it helps to prevent erosion and maintain soil moisture, making it a good candidate as a rotational crop in years when a field may otherwise be left fallow, said Cavallini. 

“Another important part of this research is that we don’t have cowpea being widely grown in Montana,” she said. “We didn’t know if the parasitoids, which are native, would be attracted to it. But we found that they were able to perceive odors from cowpea plants and move to feed on the extrafloral nectar.”

Because the experiments with cowpea were done in a lab, Cavallini said field tests are needed to determine if those results can be replicated on a farm. She added that incorporating this biocontrol could be effective alongside the development of solid-stemmed wheat varieties that are more difficult for sawflies to burrow into. As Cavallini moves on to a doctoral program at North Carolina State University, she hopes future graduate students at MSU will continue those explorations.

“Altogether, this research has the potential to have important impacts on how wheat stem sawfly is managed in Montana,” Cavallini said.

David Weaver, Department of Land Resources and Environmental Sciences, weaver@montana.edu or 406-994-7608

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