The BugLady has seen plenty of knapweed in her life, but she was surprised when she came upon a patch of it in mid-July that was covered with weevils.
If you’ve driven through Wisconsin’s Door County, you’re familiar with Spotted Knapweed. Bedrock is pretty close to the surface in the county, and many of the soils that overlay it are poor. Spotted knapweed (Centaurea stoebe), originally from the dry, Mediterranean coasts of Eurasia, is at home in Door County. Ecologically speaking, knapweed is a pioneer species, among the first to grow and build soil in bare spots, but once established, it creeps into surrounding areas. It arrived in the Pacific Northwest in the late 1890’s, maybe in a shipment of alfalfa bales or possibly as an early invader carried in contaminated ballast. It’s a pretty plant – many invasives are – and seemingly, it’s growing where nothing else wants to, so what’s the problem?
First, there are native plants that can thrive in poor soils, but not if aggressive knapweed is already there blocking the sun, sucking up the water (it has a taproot, and most of its competition doesn’t), making a lot of seeds, and living for almost a decade, threatening biodiversity. Second, Spotted knapweed releases an allelopathic chemical into the soil, an “herbicide” that inhibits growth by other plants. Third, outside of some butterflies and bees that enjoy its nectar, it’s not used much by wildlife. Its bitter leaves ensure that it is not eaten, and if livestock does graze on knapweed, its seeds can travel unharmed through their guts and be deposited elsewhere. Fourth, like other invasives, whatever kept it in check in the Old Country stayed in the Old Country. As a result, it has taken over rangelands in the West and is considered a noxious weed (no propagation, sale or transport allowed) From Sea to Shining Sea.
Riding to the rescue we have biological controls in the form of a few moths, seed flies, and beetles. Biological control has gotten more sophisticated these days (it could hardly get less sophisticated). For more than a century, we’ve imported grazers/predators of Eurasian plant and animal pests, naively thinking that the biocontrol agent would target only the species we wanted it to, but the landscape is littered with the bodies of native species that these imports went after instead. Like the imported tachinid flies that discovered early on that they favored the Giant Silk Moths (Lunas and Cecropias) over the gypsy moths they were meant to stalk. Like the mongooses (the BugLady looked it up – it’s not mongeese) that were brought to the Hawaiian Islands to kill Norwegian rats that escaped from sailing ships and feasted on the sugarcane crops – the mongooses discovered that ground-nesting birds were easier to catch. Nowadays, we do our due diligence, studying prospective candidates in the lab for a long time to make sure that once released, they’ll stick with the plan.
The weevil, she learned, was either the four-to-five-millimeter-long Lesser knapweed beetle (Larinus minutus) or the five-to-seven-millimeter Knapweed Seedhead-feeding Weevil, Blunt Knapweed Flower Weevil, or Knapweed Flower-feeding Weevil (Larinus obtusus), two weevils that look and act pretty much the same and that, said one source, may eventually be reclassified into a single species. You can tell them apart by the rostrum (snout) in pictures A and B . The BugLady thinks this is the larger species, the Knapweed Seedhead-feeding Weevil, which also specializes more in Spotted knapweed rather than other species.
There are five species in the genus Larinus (family Curculionidae) in North America, and like the knapweed plant, our knapweed-feeding weevils come from eastern Europe and the Middle East, all introduced for pest control. The KSFW was first released in the US in 1991, in northern Colorado. Its first “applications” were in the western states, but now it is the bug of choice for spotted knapweed from Oregon to Arizona to Arkansas to Rhode Island to Michigan to Minnesota, and across the southern tier of Canada. Here’s a super-close picture .
The KSFW’s life is centered around the flower head of a knapweed. Courtship happens there as the plants start to flower, the eggs are laid there, the larvae hatch, dig in, and feed on the developing seeds, they pupate there (in a case made of plant bits plus “secretions”), and the adults feed there, first on the foliage and then on the flowers. In fall, adults shelter in the dead vegetation below the plant, spending the winter there and restarting in spring.
The BugLady knew that knapweed was a troublesome invasive, but she didn’t realize that knapweed control was such big business. Knapweed is very good at what it does, but so are the weevils. Says the Cornell University College of Agriculture and Life Sciences on its website, “L. obtusus larvae destroy nearly one-hundred percent of the seeds. Any seeds that are not eaten will become a part of the ‘cocoon’. More than one larva may develop in a seedhead.” KSFWs greatly reduce the number of seeds that lurk on the ground in the seed bank, waiting to grow, and eventually another European, a Buprestid beetle called the Bronze knapweed root borer , will pick off the plants.
KSFWs may be available through a state agency near you. (Here’s the Wisconsin DNR’s brochure. ) Research shows that despite being strong flyers, they typically stay in the same area for a couple of years after release, and then spread dramatically. And they promise not to eat anything but knapweed.