Biological control of leafy spurge

In 1987, when I was news editor of the Weyburn Review, I journeyed to a small lake near Maxim to photograph beetles infesting the pretty yellow-flowered plants growing on its steep banks (hey, the news business isn’t all politicians and other disasters!).

Today, I’m told (though I haven’t had the opportunity to go see for myself), those pretty yellow flowers are gone.

An ecological disaster? Not exactly– not when the pretty plant in question is one of the most noxious of noxious weeds. In this story, the bugs are heroes, not villains.

The plants were leafy spurge, whose stems and leaves are filled with a milky latex harsh enough to cause a rash on human skin as well as blistering the mouths of cattle. That means that cattle can’t graze where leafy spurge grows, and when leafy spurge starts to grow, it grows everywhere, completely covering the ground and driving out native plants and the animals that depend on them.

Leafy spurge itself isn’t a native; it hitched a ride with the seed Eastern European immigrants brought to the prairies a century or so ago. On the steppes of Russia and the Ukraine, leafy spurge had natural enemies; here, it has none.

Or it didn’t, until Agriculture Canada researcher Peter Harris and colleagues decided to take on leafy spurge using “classical biocontrol”: seeking out a weed’s natural insect enemies and using them against it.

They started in the 1960s with defoliators, insects that eat leaves. That, in hindsight, was a mistake, Dr. Harris said; spurge puts most of its energy into its root crown, so if you get rid of its top, with defoliating insects, grazing sheep (sheep’ll eat just about anything) or herbicide (which costs more to use than the grass you want to grow is worth), it comes back very quickly.

The key turned out to be root beetles, whose larvae feed on the plant’s roots. Searching across Europe, the scientists came up with several candidates, two of which have been particularly successful: the black-dot beetle from Switzerland and the brown-dot beetle from Hungary.

Root beetles are particularly effective in our dry climate, Dr. Harris said, because the plant needs all of its roots to get enough water. As he put it, “when it loses its roots, it’s very embarrassed.”

Colonies of the black and brown-dot beetles are now being established in rural municipalities across Saskatchewan, as well as in Manitoba and Alberta and even in Ontario and Nova Scotia, where cypress spurge, closely related to leafy spurge, is a problem. As well, the U.S. took 100,000 of each kind of beetle this year.

Collecting 100,000 beetles fortunately isn’t a matter of picking each one by hand. Swiping a butterfly net across the tops of infested plants will collect several hundred, because although it’s the larvae feeding on the roots that do the most damage, the adult beetles like to eat the leafy shoots. It takes only a couple of hundred beetles to establish a viable colony.

Of course, it’s important to make sure that any new insect introduced to the province eats only what it’s supposed to, and doesn’t take a liking to wheat, sunflowers, or dogs and cats. Any candidate for biocontrol use must undergo an expensive screening process (about $400,000 per species), a process the spurge beetles passed with flying colours: they’ll starve rather than eat anything else.

The expense is one reason biocontrol is only developed for certain particularly troublesome weeds; after all, as Dr. Harris says, “If you only have one weed, the cheapest thing to do is pull it out.” Another is that it’s very difficult to develop biocontrol for weeds that are closely related to economically important crops. Anything that eats wild oats, for example, will probably also enjoy munching on cultivated oats.

The black and brown-dot beetles are only a beginning where spurge is concerned; both of them like dry, sunny spots, although the brown-dot is happy in places a little moister than the black-dot. The focus now is on the search for beetles that will attack the plant in wet, shady places with heavy soil. Some insects are already being tested and showing promise; others may soon be brought over from Europe or possibly from China, where spurge grows on a plateau in northern Mongolia that is very similar to Saskatchewan. There, Dr. Harris said, spurge is heavily attacked by a number of insects, to the point that when it springs up along the verges of new highways, it is quickly eliminated by the combined stress of insect attack and grass coming in.

“This is what we want,” Dr. Harris said. “Biocontrol is never going to eradicate leafy spurge, but we want to push it down to where it’s no longer a problem to anyone.”

I wish him luck, not only with leafy spurge but with the other 22 weeds targeted for biocontrol. And as they search the world over for beetles that eat spurge, I hope they’ll also keep their eye out for beetles that will keep grass nibbled short all summer long.

I volunteer my lawn as a test plot.

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