If you live in Delta County, chances are good you have seen Russian knapweed. If you own any land in Delta County, chances are good you really don't like Russian knapweed. You are not alone.
Russian knapweed was introduced accidentally into the United States in the late 1800s. It has no value to farmers and ranchers, and it outcompetes crops and grazing vegetation. Additionally, Russian knapweed competes with native vegetation, affecting the ecology of our wildlands. Russian knapweed is found throughout the United States but is most problematic in the open range of the west. In most instances it is already so widespread the use of herbicides is not cost effective or environmentally conscious.
Biological control of weeds is the use of natural enemies such as insects, mites, pathogens or other animals to reduce the spread, reproductive ability or density of the target weed. Extensive host-specificity testing is done with any potential biological control agent before it is released on its intended target weed. If an agent is not host-specific enough, it is not approved to be released in the U.S.
The State of Colorado is fortunate to have a large and active program that works with biological control agents on many problematic pest species in the state and primarily operates out of an insectary in Palisade. It is at this location insects are reared, collected, sorted and then distributed throughout the state. It is an impressive program that surpasses almost every one of its kind in the U.S. It is here where an ongoing biological control project for Russian knapweed takes place.
There are currently two biological control agents approved for release on Russian knapweed in the U.S. One is a stem galling fly and the other is a stem galling wasp. The stem galling fly has been reared successfully and is currently being distributed throughout the state with establishment success; however, the stem galling wasp has proven more difficult to work with and is not available for wide distribution yet.
The stem galling fly, the Russian knapweed gall fly, can complete up to four generations in a growing season, which has likely contributed to establishment success. As current biological control practices dictate, this tiny fly is extremely host-specific and will only target Russian knapweed. An adult female seeks out young, tender plant material and lays her eggs on the growing tips of the plant. When the eggs hatch and larvae begin to feed, they force the plant to form a gall around them and create a safe area to pupate inside. There can be up to 14 pupae inside one gall with that many adults subsequently emerging. This entire life cycle takes approximately 28 days.
Biological control is rarely used as an eradication method. The goal is to restore ecological balance to a system and reduce monoculture stands of the invasive plant species. That being said, the Russian knapweed gall fly will not kill Russian knapweed. What the flies will do is create galls on Russian knapweed plants that will make them smaller and produce fewer seeds. This is beneficial because smaller plants mean weaker, not-as-competitive plants and fewer seeds mean less opportunity for starting new infestations of Russian knapweed elsewhere.
Biological control is not a silver bullet solution to Russian knapweed control, but the Colorado Department of Agriculture has had success with getting gall flies distributed and established around the state. CDA has high hopes for being able to do the same with the stem galling wasp in the future as well. This method of management is not an overnight solution; it requires time and patience. It is one option within the integrated pest management scheme and is really only ideal for large infestations of Russian knapweed.
If you are interested in getting on the Russian knapweed gall fly request list or learning more about biological control agents, visit the CDA website at:https://www.colorado.gov/pacific/agconservation/biocontrol or call the Palisade Insectary at (970) 464-7916.