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.
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 other 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% respectively. Results from further field experimentation in Iran, conducted with colleagues from Mashhad University, were inconclusive.
In 2018, we moved the open-field plot to Shirvan in Iran, where Aceria acroptiloni occurs naturally. We hope field studies in this new location will prove more successful, otherwise we will have to postpone any further work with this species.
The field surveys in Kazakhstan proved extremely successful with a potential new agent being discovered - a small gall forming weevil, Pseudorchestes distans. This species has only been recorded on Russian knapweed and is thought to have a significant impact. We plan to initiate work with this species in 2019.