Ask The Entomologist, Plants

Knapweed, my current enemy.

At this time of year, many hill-slopes have turned a sharp pink-purple color. Whether you’re in Glacier or driving along 93, you’ll see its flowers in the cuts alongside the road. Here in Trego proper, you can often find it in ditches, or there’s an abundant field of it downslope from the Trego Pub. Knapweed. It’s everywhere, and looking far healthier than anyone would like it to.

Spotted Knapweed: note the black markings on the green, just below the flower blossom – these are the “spots”.

Knapweed (Centaurea sp.) is a genus of invasive plant that plagues rangeland across western North America. Its seeds made landfall on the west coast, back in the early 1900s, possibly due to contaminated alfalfa seed. Now, over a century later, knapweeds flourish across America, from sea to shining sea, more successful than they were in their homeland of southern Russia.

Thanks to the absence of the specialist insect herbivores that didn’t journey with it to the New World, and its bad taste, few things eat knapweed. And even if you are able to get something to eat it, with taproots reaching up to four feet deep, Spotted Knapweed is able to resprout with ease. Most knapweeds are only palatable to cattle early in their growing phase, quickly outcompeting grasses in overgrazed areas. To emphasize how terrible it can be in rangeland, feeding on Russian Knapweed may even prove fatal to horses.

Several common native North American grasshoppers, notably the Red-Legged Grasshopper, feed on knapweed, but they weren’t effective at controlling it. Biocontrol began in the 1980s, with introduction of knapweed-specialist weevils, flies, and wasps. Over the past 40 years they’ve proven quite useful – if there are enough seed-head feeding insects, many fewer knapweed seeds are produced each year. It’s still not a quick fix – knapweed seeds can lay dormant in the soil for at least seven years.

The Knapweed Peacock Fly: introduced for biocontrol, its maggots destroy knapweed seedheads.

These introduced bugs have done wonderful things. Knapweed is much less a problem than it once was, but biocontrol alone isn’t enough. If you happen to have knapweed on your property, the best thing you can do is to start spraying herbicide. I’ve been having decent success eliminating it with Milestone (Aminopyralid).

As best I can tell, knapweed’s one virtue seems to be a high rate of nectar production – some folks enjoy making knapweed honey.

What are your thoughts on knapweed? Have you ever tried knap honey?

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