Soybean School: Painted lady caterpillar dines on thistle and soybeans

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The painted lady caterpillar, also known as the thistle caterpillar, is typically something pulse growers in Western Canada have seen as a beneficial insect.

This is because it feeds on Canada thistle.

However, as seen in 2017, the painted lady caterpillar also likes to chew on soybeans, causing leaf damage and potential yield losses.

In this Soybean School episode, Glenda Clezy, agronomy specialist with Saskatchewan Pulse Growers, explains as soybean acres increase across Saskatchewan, they’re starting to see different issues arise — one of these being the painted lady caterpillar.

“It will feed off the foliage, so there is some leaf damage that we can see. We don’t have a lot of research that’s been done specifically in Saskatchewan in terms of economic threshold, but if we look at some of the results in the U.S., during the flowering stage, what they are finding is that around 25 per cent foliar damage is about an economic threshold you may want to consider looking at,” she says.

While no specific thresholds have been developed, Manitoba Agriculture has a suggested threshold of 25 to 30 percent defoliation prior to bloom and 20 percent after bloom or pod set.

Painted lady butterfly (Fritz Geller-Grimm, CC BY-SA 2.5, Wikimedia Commons)

Because the painted lady prefers thistle, it’s important that growers keep in mind the benefits it brings, as well as the benefits of other insects that would be affected by an insecticide application.

“It’s important to just really look at the amount of damage that you have and if it’s economic to spray or not, and to take into consideration things like your beneficial insects that you may be controlling at the same time. So you know the thistle caterpillar will feed off of the Canada thistle, which makes it a good pest in those crops,” Clezy adds. “But we certainly don’t want to see yield loss in your soybeans, so it’s really considering what beneficials you might impact if you are spraying for it, versus if it’s not going to cause a lot of damage.”

This is the first year the caterpillar has been a notable pest in soybeans in Saskatchewan, but as the soybean acres increase in Western Canada, it is likely to be noticed more, she says.

Painted lady butterflies usually migrate into Western Canada from California and Mexico in June, often laying eggs on Canada thistle plants. The painted lady caterpillar will also occasionally feed on sunflowers and canola.

To learn more about the painted lady or thistle caterpillar, check out our latest Soybean School filmed at Ag in Motion near Langham, Saskatchewan:

Watch more Soybean School videos here.

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