One prairie farmer hopes that butterflies may soon become a viable weed management tool for organic producers, but insect experts caution it could be a risky business.
Bob Willick, a certified organic farmer from Blaine Lake, Sask., said painted lady butterflies effectively kept down the thistles in his field peas this growing season.
“The peas were growing and the caterpillars were eating the thistles. I thought this crop, I might even have to plow it down. I kind of took my chances on it. And the way it turned out, the peas grew fast enough that I got a real nice crop.”
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Willick said he first noticed the butterflies on July 12, when an organic inspector made her annual visit with Willick to his pea field. He said they discovered patches of dead thistles and webs that they later attributed to the hungry butterfly larvae.
“My first reaction was there’s something fantastic happening here, and then I started thinking geez, maybe (the inspector’s) going to think I sprayed it, because there were so many brown leaves there.”
The painted lady butterfly begins its life cycle as an egg laid in thistle, mallow or hollyhock. After an incubation period of three to five days, the emerging yellow-green striped, purple to black larva eats continually for five to 10 days before pupating and becoming an adult.
The adult painted lady butterfly is known as the thistle butterfly and is the most widespread in the world. It is mostly black, brown and orange with some white spots; the underside is grey with white and red markings.
Entomologist Bob Randell said he has noticed higher than usual populations of painted ladies this year, although he isn’t sure of the cause.
“The things that we don’t know about insects are about 10 times as much as what we do know.”
He said that all major damage done to crops by winged insects happens when they are in their larva or caterpillar stage.
Willick would like to see a farmer-driven organization promote and manage painted lady populations as a biological weed control, importing eggs from their winter home in the south, if need be. He plans to bring the issue up at the next meeting of the research committee of the Saskatchewan Organic Directorate, of which he is a past vice-president.
“Successfully growing good peas in organic farming is a challenge, and we’re going to have to see if this over the long haul is going to prove to be a way to get the competitive advantage for our crops, so that we end up with good results like we got this year.”
However Scott Hartley, insect specialist with Saskatchewan Agriculture, said although using the caterpillars as a widespread method of weed control is worth looking into, producers would have to be assured there were sufficient thistles present in crops to make it worthwhile.
He said because the butterfly larvae aren’t as weed specific as other biological controls, it wouldn’t be a recommended practice in most cases.
“It could create some grief.”
Hartley said there’s a fine line between when the butterfly larvae function as a biological weed control and when they become a pest.
“There’s an advantage to them, but you just have to sort of watch it,” he said. “If they’re confining themselves to the thistles … obviously that’s beneficial. Let them be.”
He said if after eating the weeds the caterpillars are still hungry, they will move onto an alternative food source: the crop.
Control measures against the butterfly larvae as a pest have been needed in some areas this year due to their higher than normal populations, Hartley said.
The painted lady caterpillars can be troublesome for canola, sunflowers and borage, he said. The bacterium Btk (bacillus thuringiensis subspecies kurstaki) is used to kill the larvae.
While Randell said painted lady larvae are distinct in appearance from other caterpillars, Willick suspects some producers have sprayed crops thinking they were infested with Bertha armyworms, when in fact they were the less harmful painted ladies.
“That’s a very common thing for people to mistake insects for other insects,” Hartley said.
He urged farmers to discover exactly what insects are in their crops and determine if serious damage is being done before applying insecticides.
