Diggings logo

WSU/Pend Oreille Extension introduced the Sense of Place series in 1999, with a focus on local landscape and natural history of Pend Oreille County, Washington. A partnership with the Kalispel Tribe of Indians Natural Resources Department has allowed us to expand this program through EPA funding to include more classes, a newsletter and this website.

Spotted Knapweed

What changes to our forest does the ubiquitous spotted knapweed (Centaurea biebersteinii) bring? You won’t find it in deep shade, but it readily invades into the forest along roads, slowly encroaching into the surrounding openings, even into areas with healthy native cover of grasses and forbs. Knapweed can increase erosion and decrease wildlife forage and habitat. A study from Montana shows that knapweed infestations can also suppress establishment of Ponderosa pine. Seeds are large and heavy, and each mature plant is capable of producing in excess of 25,000 seeds that at last count, can survive 20 years in the soil.

If you take an old knapweed seed head and carefully break it open, what will you see? Some heads will be full of old seeds from last fall, but many will contain something else. Three kinds of insects (the knapweed seed head moth, the UV seed head fly and the banded knapweed gall fly) were released in the northwest 20-30 years ago to help control the spread of spotted knapweed. They all lay their eggs on the knapweed flowers and when the immature insects (larvae) hatch, they feed on the newly developing seeds. These small larvae overwinter inside the seed heads and then in the late spring they pupate and emerge as adults. All three insects are so widely distributed now that you can find them anywhere… you just have to start opening up old seed heads.

In certain select places in this county you might see a small round hole cut out of the middle of the seed head. This is the emergence hole of the knapweed flower weevil. In addition to larval seed destruction, the adults can severely defoliate a plant. Approximately ten thousand weevils have been released in the last several years and almost as many are scheduled for release by the end of this summer. Two species of seed feeding weevils have been released in Pend Oreille County in hopes of establishing nursery colonies which then can be used to rear even more insects to distribute throughout the county.

So by now you’re wondering why, if all these insects are attacking the knapweed, it seems like every year there’s more knapweed, not less. The answer lies in the biology of knapweed and the history of the biocontrol of knapweed program. For many years, scientists believed that spotted knapweed was a biennial (living only two years), which led them to focus on seed reduction. The realization that knapweed can live much longer prompted

additional tactics. So more recently the program has also focused on distributing root feeders that can damage young plants enough to kill them. In Pend Oreille County, we have made a few small releases of some root feeders in the past. With the support of a grant from the Forest Service this year we will be able to dedicate much more time to the evaluation of previous releases and the establishment of more nurseries

<----previous article...Diggings Aug 2003...next article---->