Russian knapweed has two approved biological control agents in California: the gall wasp, Aulacidea acroptilonica (Hymenoptera: Cynipidae), and the gall fly, Jaapiella ivannkovi (Diptera: Cecidomyiidae). The stem gall nematode (Subanguina picridis) has been released in some other states, but is not very effective, and is not permitted for release in California.

The gall wasp has one generation per year. Adults appear in the spring when plants are emerging from the soil (Fig. 1). About 90% of the insects are female, but adults live for only about 5 days (range 2-9 days). Females deposit eggs in the young growing stems, and the hatched larvae cause the formation of a stem gall (Fig. 2). Larvae remain inside the gall all winter, and adults emerge from exit holes the following spring (Fig. 3). The gall wasp has established good populations at several locations in Lassen County and at least one location in Siskiyou County. Efforts are now underway to release the gall wasp on Russian knapweed infestations throughout California, especially the San Joaquin Valley. Galled plants produce few flowers and fewer seeds. Post-release monitoring in 2019 showed that the gall wasp caused a 66% and 85% decline in seed production at sites in Lassen and Siskiyou County, respectively.

The adult female gall fly deposits eggs on buds at the tip of stems (Fig. 4), and the larvae induce the formation of a 'rosette gall' comprised of stunted shoot and bunched leaves at the stem tip (Fig. 5). Up to 14 larvae can occur inside one gall. Adult gall flies emerge in spring when the plants are emerging from the roots. Adults live for about 3 days (range (2-7 days), and the sex ratio is 1:1. There are multiple generations each year, with the last generation overwintering as larvae in the galls. Despite releases of several hundred insects from 2011 through 2015, the gall fly has not established in California. This is due, in part, to the fly's requirement for a second generation in summer. In California, Russian knapweed plants stop growing in June or July due to lack of moisture, and the absence of new growth in the summer causes the gall fly to die out because it is unable to form galls.

Biological Control Agents


Common name




Aulacidea acroptilonica

gall wasp



First released in CA in 2014

Jaapiella ivannkovi

gall fly

failed to establish


First released in CA in 2011

How the Technique Is Employed

Galls can be collected in later winter or early spring and moved to new field sites. However, at the release site in Siskiyou County, overwintering gall wasp larvae experienced an infestation rate of 9% by local parasitoids (parasitic wasps). It is best to not move parasitoids to new release sites, so collect galls from field sites in early spring and hold them indoors for adult wasps to emerge. The parasitoids, which are cigar-shaped, have a greenish sheen and are slightly smaller, usually emerge a week before the gall wasps, which are larger and have shiny black, more bulbous abdomens. The gall wasp needs young growing plants to produce galls so timing of the release is critical. Establishment success is highest in early spring when plants are between 1 and 5 inches in height. Once a plant has reached full height and begins to flower, the plant is no longer putting out new growth, and it is too late for the wasp to initiate gall formation. The best time for releases is March for plants in the San Joaquin Valley and April for plants in northern California. The gall wasp should be released where immediate eradication of Russian knapweed populations is not the primary objective.

Special Tips

Russian knapweed is a common pest of pastures and grazing lands; however, cattle destroy the galls during grazing, and the gall wasp does not persist. Fencing a small area within a pasture to prevent grazing has allowed the gall wasp to establish and build up populations quickly.

The gall wasp is present in plants throughout the year. Adult gall wasps emerge in early spring and are active visiting young growing shoots in March (central CA) and April (northern CA), while larvae are present inside stem galls during the other ten months. Dead plants with galls laying on the ground have live gall wasp larvae, so it is best not to remove these plants until adults have finished emergence.


Herbicides that kill Russian knapweed plants are usually applied during early spring when adult wasps are active. It is best not to use herbicides where the gall wasp is released and being relied upon to control plants. Note that the biological control agent will probably not establish in areas where herbicides are used regularly, such as road shoulders.

Grazing can also impact the effectiveness of gall wasps (see Special Tips).

Where Can I Get These?

This gall wasp is a part of a new distribution program by the California Department of Food and Agriculture (CDFA) and is available depending on supply. To obtain insects for release contact your local Agricultural Commissioner who will contact CDFA and request for a release.

To date, there are no known commercial vendors of the gall wasp.


Djamankulova, G., A. Khamraev, U. Schaffner. 2008. Impact of two shoot-galling biological control candidates on Russian knapweed, Acroptilon repens. Biological Control 46: 101-106.

Pitcairn, M.J., V. Popescu, B. Villegas, J. Aceves, and C. Gibbs. 2019. Biological Control of Russian knapweed in northern California: release of gall midge Jaapiella ivannikova (Diptera: Cecidomyiidae). In: C.H. Pickett (ed) Biological Control Program 2018 Annual Summary, California Department of Food and Agriculture, Plant Health and Pest Prevention Services, Sacramento, CA pp. 33-36.

Pitcairn, M.J., V. Popescu, J. Littlefield, T. Getts, and J. Aceves. 2019. Biological Control of Russian knapweed in northern California: release of gall wasp Aulacidea acroptilonica (Hymenoptera: Cynipidae). In: C.H. Pickett (ed) Biological Control Program 2018 Annual Summary, California Department of Food and Agriculture, Plant Health and Pest Prevention Services, Sacramento, CA, pp. 37-41.

Contributing Authors

Dr. Michael J. Pitcairn, Program Manager, California Department of Food and Agriculture

Dr. Lincoln Smith, Research Entomologist, USDA