A tiny pest known as the wheat stem sawfly can lay a wheat field over in short order, and it costs farmers a bundle of money every year. In Montana alone, producers lose upwards of $25 million to this native villain, which, of course, is not stymied by any artificial geographical lines. The pest hurts producers in Northwestern North Dakota just as much as producers in northeastern Montana.
Among the researchers in the MonDak Research Triangle studying this pest is research ecologist Dr. Tatyana Rand, with the USDA-ARS unit here in Sidney. Rand spoke about her work on wheat steam sawfly, as well as alfalfa weevils, during the MonDak Ag Days.
One of the things that makes the wheat stem sawfly so difficult for producers to manage is that the pest spends so much of its lifecycle inside the wheat stem, where it is largely protected from pesticides. And, even when the sawfly comes out, it does so over such a wide window of time, that it’s difficult to control withh a pesticide. Each female can lay 50 eggs or so, which means it doesn’t take a very large survival rate to still have a lot of sawfly. These factors make biological control particularly attractive in this case.
Rand has looked at a variety of things that might help defeat the wheat stem sawfly over the years, and she is particularly interested in its natural predators, braconid wasps, which are nearly as tiny as the sawfly itself.
Bracon cephi and B. lissogaster have a very effective attack on wheat stem sawfly. They lay eggs in the pest larvae, paralyzing them. This prevents the sawfly from eating the wheat stem.
Left to its own devices, the sawfly will eat the wheat stem out, laying the plant over before it can be harvested. But if the wasp gets to the sawfly in time, the wheat will be saved and can continue to stand tall.
Some wheat fields are lucky enough to have a lot of these little wasps flying around, while other wheat fields have practically none at all. Figuring out why some fields have wasps and others don’t is among the many research projects Rand has taken on while working at the USDA-ARS research lab in Sidney.
Rand suspects some fields may lack these beneficial wasps because they lack sources of flower nectar to energize the wasps. It takes a lot of energy, after all, to fly around a field looking for just the right host for its eggs. Nectar snacks can keep the wasps going.
Goldenrod, sunflowers, and other such native plants might be the key to attracting beneficial insects that can help farmers keep the sawfly population in check in their wheat fields.
Other issues that affect parasitoid wasp populations include management practices, such as not cutting the wheat too low to the ground and minimizing tillage. The wasps live in the bottom two-thirds or so of the wheat steam.
Greater stem solidity for wheat also helps the plant resist sawflies in general, but these varieties may also resist the parasitoid wasps as well.
Rand also talked about her work with alfalfa weevils, an insect pest that may have some resistance to pesticides.