A good parasite doesn’t kill its host, but a good parasitoid does.
What’s the difference and why? In this episode of the Pests & Predators podcast, host Shaun Haney is joined by Tyler Wist, research scientist and field crop entomologist with Agriculture Agri-food Canada at Saskatoon, Sask., to explore the role of parasitoids in field crops.
Since 2016, Wist has been researching field pests that attack crops, and the things that attack those pests, such as parasitoids. Parasitoids are a specialized predator, whose offspring will kill the pest insect. A female parasitoid will lay eggs in the pest insect, and when they hatch, will kill the host.
For example, a tell-tale sign that a parasitoid has attacked aphids in your field is finding aphid mummies. They look like aphids, but are swollen, brown, and don’t move when you poke them. An aphidious wasp uses her antennae to find the back of an aphid, then uses her ovipositor to lay an egg inside the aphid. Within eight to ten days, the aphid dies and the larvae from the wasp feeds on the aphid body.
“If you’ve got aphids in your crop you probably have a parasitoid as well,” says Wist. If you’re in a wheat field or any kind of cereal field, you’ll have two different species of aphid, and usually one type of parasitic wasp. A similar type of parasitic wasp can help us out with aphids in peas, lentils, or faba beans. Other wasps, in a different genus, will make a different looking mummy out of the aphid body. (more after the player)
Another parasitoid you might find in wheat or barley is cotesia, which attacks wheat head armyworm. Cotesia will lay one egg but that egg divides, 10 to 200 times depending on the species, inside the body of the armyworm caterpillar. The wasps that have emerged will spin a communal cocoon, that looks like a puffball. Wheat head armyworm isn’t really a problem in fields on the Prairies because of cotesia.
Wheat midge parasitoids — Macroglenes penetrans — are hard to find but can be caught with a sweep net four or five days after the wheat midge have emerged. Those parasitoids will probably emerge in whatever field was last year’s wheat field, probably canola. So consider not spraying canola at flowering with insecticides if the beneficials are present because that’s where they will be nectaring, says Wist.
If you do think you have an issue with a pest insect, make sure you’re at economic threshold, and don’t spray unless you have to. Ask your agronomist to identify the beneficial insects that might be there and find a product that is easier on them, whenever possible.
Encouraging beneficials isn’t too hard, but they’ll need a food source, Wist says. So an unproductive area in a field, a slough edge, or a corner can be seeded to wildflowers to serve as an alternate habitat for parasitoids.
Check out the cereal aphid manager app from AAFC that’s really handy for identifying parasitoids. “When you’re scouting for aphids, you’re also scouting for beneficials too,” says Wist. It’s also a good tool for keeping track of aphid populations week after week. The app not only accounts for the economic threshold of the pest insect, but also a dynamic action threshold, by including the benefits of a parasitoid or other beneficial insect.