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Improving lives by solving problems in agriculture and the environment

Controlling noxious Russian knapweed in the North America

Russian knapweed is one of several invasive plants of rangelands that arrived in North America as a seed contaminant in the 19th century, in this case from Asia. Biological control is often a good approach for these plants, but a nematode species introduced in the 1970s proved ineffective against Russian knapweed. Funded by a US and Canadian consortium, CABI has been tasked with researching new biological control agents for introduction, some of which are already showing promise.

Project Overview

So, what's the problem

Russian knapweed (Rhaponticum repens) is an herbaceous perennial plant native to Asia. It was accidentally introduced into North America in the late 19th century as a contaminant of alfalfa seed. The plant has a large native range but genetic studies suggest that the origin of the populations in North America is Kazakhstan and/or Uzbekistan. To date, Russian knapweed has spread to 45 of the 48 contiguous states in the USA, and is considered noxious in 16 western states and one Canadian province.

What is this project doing?

In the 1970s, efforts to control Russian knapweed in the USA by biological means led to the release of a nematode species, but this agent had little impact. Investigations on biological control of Russian knapweed were resumed in 1997. CABI's centre in Switzerland has surveyed various regions in the native range to assess the herbivores associated with Russian knapweed, and has studied the biology and host specificity of selected biological control candidates. 

Results

So far, two new biological control agents have been approved for field release in the USA and Canada: the gall wasp Aulacidea acroptilonica in 2008 and the gall midge Jaapiella ivannikovi in 2009. Both species have successfully established in both countries, and a large distribution programme covering eight US states has been initiated by USDA-APHIS. In Wyoming, the gall midge reduces seed output by more than 90% and shoot biomass by 30%, confirming results of earlier impact studies in Uzbekistan. 

Current work focuses on the mite Aceria acroptiloni, a flowerbud-galling eriophyid mite with more impact than any specialist herbivore found on Russian knapweed so far. In field studies in Iran, mite attack reduced shoot biomass by 40–75%, and number of seed heads and seeds by 60–80% and 95–98%. 

Further field experimentation in Iran, conducted with colleagues from Mashhad University, has had mixed fortunes: no control plants showed signs of mite attack in 2017, thus invalidating results for non-target species. Unexpectedly too, A. acroptiloni was found on individuals of distantly related plant species, which contrasts with laboratory results and with surveys assessing the mite’s realized host range in Iran. While we continue to investigate host specificity, we also need to investigate the biology and overwintering requirements of A. acroptiloni to assess possible limitations of introducing it to North America.

The team

Project Manager

Staff image of Philip Weyl

Philip Weyl  Research Scientist, Weed Biological Control

Rue des Grillons 1 CH-2800 Delémont
Switzerland
T +41 (0)32 421 48 76
E p.weyl@cabi.org

Project team

Staff image of Urs Schaffner

Urs Schaffner

Head Ecosystems Management