When the Enemy of Your Enemy is a Friend – to Crops
Adult wheat stem sawfly on a stem of wheat (Photo courtesy of Kelsey Dawson)
Sometimes the solution you need is right in your own backyard – or near it. Farmers who grow wheat and other grains have struggled since at least the 1800s to tame the wheat stem sawfly (Cephus cinctus), an insect that gets inside grain stalks and topples them, reducing yields. A variety of defensive approaches, from pesticide to crop rotation, have had only limited success. Now, scientists at USDA’s Agricultural Research Service (ARS) are exploring a solution that leverages the sawfly’s natural foe to significantly reduce the harm it causes. If it works, it could become a critical part of the integrated pest management (IPM) approach that ARS researchers use to tackle agriculture’s insect enemies.
Many crop pests are difficult to mitigate because they are invasive, and as a result, have few (if any) natural enemies in the areas they infest. The sawfly, however, is native and as a result, has a natural enemy lurking right beside it: the parasitoid Bracon cephi is a tiny wasp, less than a quarter-inch in length, that feeds exclusively on the sawfly. The female wasp inserts an egg, through the stem of a stalk of grain, onto a sawfly larva. The resulting wasp larva then consumes the sawfly larva, reducing the insects’ numbers and impact.
The fact that this parasitic relationship already exists naturally in some areas leads researchers to believe it can be replicated in other regions the sawfly has infested.
“Typical biocontrol programs require bringing in non-native insects and trying to get them to establish in a new area,” said Erika Peirce, a remote sensing specialist at the Rangeland Resources & Systems Research Unit in Fort Collins, CO. Peirce is a member of a cross-state collaborative team made up of stakeholders and scientists that is working together to tackle the sawfly problem. “This insect has been found in Colorado since we found sawfly in the 1800s,” she said, “so we have a good foundation for believing that it will be able to establish.”
A Bracon cephi larva consuming wheat stem sawfly larva in the stem (Photo courtesy of Erika Peirce, ARS)
Although some wasps are present in Colorado, the timing of their lifecycles has not always aligned with that of the sawfly in a way that makes them effective at controlling the pest. For that reason, the researchers are looking at wasp populations from neighboring areas, which have developed in ways that make them better poised to prey on the sawfly in wheat.
The team has devised a simple yet ingenious way to transport the wasps. They call it the “beneficial bug baler,” and it’s exactly what its name suggests: a large bale (or bales) of straw, taken from areas where the wasp is widespread, and filled with wasps. The bales are relocated to areas with sawfly infestations that are not currently being controlled by wasps. There, researchers can monitor what happens in real-life field situations, and how each insect fares.
“It’s almost impossible, if not impossible, to get a population of sawfly in the lab, so we have to do this all with field-collected specimens,” explained Peirce. “That’s why the bug baler is exciting, because we found a really large population and now, we can move them to new locations.”
“It’s been really interesting to see how excited growers are about moving bales,” added Tatyana Rand, a research ecologist at the Pest Management Research Unit in Sidney, MT, who is also working on the project. “When farmers actually see something that they can do themselves, there’s something about that that’s extremely attractive. I feel like we’ve gained a lot of traction because of that. Everybody wants a bale. They just want to see if it will work. You have people really excited to try out what you’re offering.”
With careful examination of wheat straw, the scientists were able to determine that they were only moving beneficial parasitoids in their bales, a check that will need to be part of their vision for an integrated pest management approach to sawfly control. The next step is to figure out how to create the most favorable environment for the parasitoids – “so we can have larger populations that are really controlling our sawfly infestations,” said Peirce. The team is now working with the idea of creating local nurseries for the parasitoids, to avoid depleting the currently abundant populations in Nebraska that have provided their samples so far.
While there are still more questions to explore, the tiny wasp next door is turning out to be an important ally for one of America’s biggest crops. – by Kathryn Markham, ARS Office of Communications