Biocontrol – Trego's Mountain Ear

Last Thursday I saw this lovely Ichneumonid wasp (pronounced ICK-new-mon-id, from Greek “Ιχνευμων” which means “Tracker”). Most of the time I see Ichneumonids, they’re on the sides of trees, ovipositing (laying eggs) in boring insect larvae. This one’s behavior was very odd indeed.

An Ichneumonid wasp, **Pimpla pedalis**, oviposits into a newly split piece of Douglas Fir.

As you can see, this particular Ichneumonid wasp was laying her eggs inside a freshly split piece of Douglas Fir … or rather, inside a boring insect inside the Douglas Fir. Curious to see what insect she was laying her eggs inside, I peeled away layer after thin layer of wood …

Uncovering the beetle grub (at left, mid-height) the wasp laid her eggs in.

… And after an inch and a half of wood was removed, exposed a boring beetle grub. This is a Jewel Beetle grub, a member of family Buprestidae. These beetles can be lumber pests, though they’re unlikely to damage treated wood. While none of our Montanan Jewel Beetles are quite as bad, the Emerald Ash Borer has been devastating to ash trees throughout eastern North America.

There it is, a Buprestid beetle grub, just to the left of the burrow it gnawed in the wood.

I have very fond childhood memories of Giant Ichneumonid wasps. Most Sundays, my family would go to the arboretum of South Dakota State University’s then-public gardens. Among my favorite things there were some large multi-trunked cedars, which, in autumn, attracted some very large wasps. Presumably, the cedars also had very large wood-boring larvae that the Ichneumonids were parasitizing. Despite being a typical small human, making noise, climbing trees, and being generally bothersome, the Ichneumonid wasps never showed any sign of interest in me.

While their large stingers and stinger sheaths look quite formidable, Ichneumonid wasps very rarely sting mammals or other large animals. Unlike typical colony-living wasps and bees, Ichneumonid stingers are almost exclusively used for laying eggs inside of host insects. Eventually the eggs hatch, and the baby wasps eat the host insect from the inside out. Parasites that always kill their hosts are called “parasitoids“ (think of the Xenomorphs from the Alien movies).

Fortunately for us, in addition to not stinging us or our pets, Ichneumonid wasps are also great at controlling garden pests. They take out a variety of garden pests (tomato hornworms, cabbage worms, etc.) as well as lumber pests (long-horned beetles, jewel beetles, bark beetles, etc.).

All in all, they’re neighbors I’m quite glad to have.

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?

Tag: Biocontrol

Ichneumonid wasps, imposing allies

Knapweed, my current enemy.