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Weber County

Biological Control Methods for Diffuse Knapweed

Weber County, like most other weed species here, are located in a variety of surrounding habitat. Some Knapweed can be treated with herbicides very effectively and safely. Other infestations for various reasons are not suited for any kind of chemical weed control. The proximity of water or desirable plants make herbicides a liability rather than a solution.

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Biological control in these instances is a good alternative. In co-operation with USDA APHIS and the USU Extension Service, Weber County became a participant in the Cutting Edge of weed science.

The Upper Ogden Valley features grand scenic vistas as well as some pristine recreational areas. Among them is the Weber School District's Environmental Center. This venue is used year round to facilitate nature studies at several grade levels. Next door to the center is a church owned campground. Both of these properties are prime habitat for alpine-like trees, brush and wildflowers. The area has also been invaded by Diffuse Knapweed which, if left unchecked, would crowd out many of the native plants that make up the rich botanical tapestry found here. Little weed work was done on the small but growing infestation of knapweed until 1994 when a solution seemingly appeared form nowhere. A letter from the local USDA APHIS office arrived at the weed department late in 1993. We were asked if we had any Diffuse Knapweed in the county that might meet the prerequisites for a release of some host specific insects for biological control of the weed and a five year study of same. We were elated and answered back in the affirmative right away.

Dawn Holzer from APHIS, James Parks and James Barnhill from the USU Extension Service worked together for several months prior to the release on weed surveys, property owners permissions and agreements for a "no chemical zone." All that having been accomplished, a few hundred Urophora quadrifasciata were finally released in the summer of 1994. The results have been very satisfactory with a knapweed reduction in 1998 of 50% to 70% from the original stand pictured above. Since that first release, additional U. quadrifasciata were imported the following year as well as 3 or 4 other species of insects. Since that time, after a hatch, the area is rich with seedhead fly on the wing.
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Macro Photography is not a weed department specialty. Even so, we include this photo (far right) of one of our own adult U. quadrifasciata which are now quite easily found on the North Fork Knapweed .... so much so that APHIS and other county weed departments come here to collect the adult seedhead fly along with the other insects thriving on the Diffuse Knapweed remaining at the Environmental Center.

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Another Weed Feeder introduced on Weber County Knapweed is a root boring beetle, Sphenoptera jugoslavica. Pictured to the left is the larva of the S. jugoslavica in the typical gall-like swelling of a Diffuse Knapweed root crown. In 1998, a whopping 90% of the adult Knapweed root crowns we cut open had the fat weed feeder larva inside doing its damage by stunting the plant and causing far less flowers to bloom.

Field Days at this site hosted by Weber County have drawn as many as 30 participants from 3 states who come to spend the day collecting weed feeding insects. At dusk, the moths that collectors have come for are attracted by a special light. The flies and beetles are gathered in the daylight with canvas "grabbers" as seen in the lower trilogy of photos, left. The collector walks through the stand of Knapweed swinging the grabber side to side. In a few passes the bag will have gathered a variety of things, the seedhead fly being among them.

In the center photograph, Craig Searle of the Utah County Weed Department sorts through his collected bounty with a suction apparatus. Even with his high fashion shades in place, his keen eye finds several of the quick U. quadrifasciata among the treasures dumped from the bag, and collects them in the suction vial.

The third photo of the group below, right, Shows Steve Burningham and Grand County Weed Supervisor Tim Higgs admiring a vial of collected seedhead flies. By any measure, the field day was a success, as is the entire ongoing experience with biological control on the knapweed in Northeastern Weber County. If any of this has had interest to you as a possible solution to your weed challenges, we hope that you will take a moment and follow some of the hyper-links found on this page and get some additional information from those people and agencies mentioned.


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