Four agents have been released in California, and at least three of them have become established: a gall midge, a gall mite and a rust fungus pathogen. The fungus and the gall mite appear to have the most impact, although the latter is often limited by naturally-occurring predators. The root moth was first released in 2014 but has not been recovered at its release sites. The weed is generally well controlled in California by biological control agents. No non-target plants appear to be at risk of attack.

Gall mites (Aceria chondrillae, Acari: Eriophyidae) are microscopic and can be seen at 20x magnification (fig. 1). Females form galls on axillary and terminal plant buds in which hundreds of larvae can develop (Figs. 2). Mites develop rapidly (in 10 or more days) and can have many generations per year. They probably disperse by wind, like pollen. Mite infestation reduces plant vigor and reproduction, and high infestations can kill young plants (Fig. 3). Adults overwinter in rosette shoot buds. Mites are most successful at warm sites (S or SW exposure) with well-drained soil and little soil disturbance (not on cultivated croplands). The mites tolerate high summer temperatures (95°F), but severe winter conditions in Idaho appear to reduce survival. Predaceous mites that feed on gall mites can limit their effectiveness.

Larvae of the root moth (Bradyrrhoa gilveolella,Lepidoptera: Pyralidae) eat internally and externally on the roots and hide in tunnels comprised of silk, frass and soil particles (Fig. 4). There are 1-2 generations per year in Oregon. In Europe, adults emerge in May to June and August to October (Fig. 5). Larval damage exposes roots to soil pathogens, and attack by multiple larvae can kill above-ground plant parts.

Larvae of the gall midge (Cystiphora schmidti, Diptera: Cecidomyiidae) develop inside small galls (1/8 in. diameter) on the leaves and stem (Figs. 6-8). Pupation occurs inside the galls or on the soil surface. There are 4-5 generations per year. Adults are active from April to October. Infestation reduces plant growth and reproduction, and high infestations can cause plants to die. Gall midges are most abundant where the average yearly temperature is higher than 63°F and precipitation is less than 400 mm (16 in). Heaviest attacks occur at open locations with well-drained soil. Note that native parasitoids have greatly reduced effectiveness of this agent in California.

The rust fungus (Puccinia chondrillina, Uredinales: Pucciniaceae) develops on leaves and stems creating pustules that release spores (Fig. 9). Infection reduces plant vigor, reproduction and survival. Infection of rosettes in fall and spring often kills plants. The rust appears to be effective on 2 of the 3 biotypes of rush skeletonweed in the USA, including the one known to occur in California. It develops best at more humid sites. In a field experiment, infested plants had 89% less biomass and produced 94% fewer seeds than uninfested plants, and 65% of plants died prematurely (Emge et al. 1981).

At three field sites in central California, the combined impact of the three biological control agents (gall midge, gall mite and rust pathogen) reduced the density of skeletonweed plants between 56% and 87% (Supkoff et al.1988).

Biological Control Agents


Common name




Aceria chondrillae

gall mite



First released in CA in 1977

Bradyrrhoa gilveolella

root moth

not recovered in CA


First released in CA in 2014

Cystiphora schmidti

gall midge



First released in CA in 1975

Puccinia chondrillina

rust fungus



First released in Ca in 1976

How the Technique Is Employed

Some biological control agents are likely to be present at your site. Look for signs of their damage. Gall mites distort the flower buds into gall tissue (Fig. 1). The root moth larvae produce tunnel damage in the roots (Fig. 2). The gall midge produces lumps on stems and leaves (Fig. 3). The rust fungus produces rust-colored clumps of spores on leaves (Fig. 4).

For the gall mite, collect galled stems July-Oct. and place them in direct contact with target plants so that mites can crawl onto them. Galled stems can be refrigerated for up to several weeks before releasing mites.

Root moth adults can be collected by sweep net during the evening in May-June, keep them cool, and release them as soon as possible.

For the gall midge, collect galled stems from early July to late Sept. Remove seed heads and flowers to prevent distributing seed. Tie the stems together to form a tipi and place them among the target plants so that adults can emerge and lay eggs. Note that this method may inadvertently introduce native parasitoids, which have greatly reduced effectiveness of this agent. It would be best to allow the insects to emerge inside a cage and then separate the midges from parasitoids (Hymenoptera) so that only midges are released.

Rusted stems can be collected in summer and placed at target sites to release teliospores in the fall. Spore germination requires a long dew period (8-16 hours), so cool humid evenings are optimal. Rosettes with pustules (uredia) can be dug up and transplanted to target sites in spring or fall.

For more details, see Milan et al.(2016).

Special Tips



Rush skeletonweed has least three different biotypes in North America that vary in resistance to the mite and the rust fungus. All known biotypes in California are susceptible to the mite and to the rust.

Native natural enemies have limited the effectiveness of the gall midge (parasitoids) and the gall mite (predatory mites). Transferring galled plant material can easily transfer these natural enemies, if they are present, so this should not be done.

A naturally-occurring parasitic fungal disease has been reported to reduce effectiveness of the rust fungus (P. chondrillina)in Idaho. Therefore, do not bring rush skeletonweed plant material from other states to prevent introduction of other plant diseases that may interfere with effective biological control in California.

Where Can I Get These?

Some agents may be available from your county Agricultural Commissioner.


Emge, R.G., J.S. Melching and C.H. Kingsolver. 1981. Epidemiology of Puccinia chondrillina, a rust pathogen for the biological control of rush skeletonweed in the United States. Phytopathology 71: 839-843.

Gaskin, J.F., M. Schwarzländer, C.L. Kinter, J.F. Smith, and S.J. Novak. 2013. Propagule pressure, genetic structure, and geographic origins of Chondrilla juncea (Asteraceae): An apomictic invader on three continents. American Journal of Botany 100: 1871-1882.

Milan, J., C.B. Randall, J.E. Andreas, and R.L. Winston. 2016. Biology and Biological Control of Rush Skeletonweed. Forest Health Technology Enterprise Team, U.S. Forest Service, FHTET-2016-05.

Piper, G.L. and L.A. Andres. 1995. Rush skeletonweed. In: J.R. Nechols, L.A. Andres, J.W. Beardsley, R.D. Goeden and C.G. Jackson (eds.), Biological Control in the Western United States: Accomplishments and benefits of regional research project W-84, 1964-1989. University of California, Division of Agriculture and Natural Resources, Oakland. Publ. 3361. pp. 252-255.

Piper, G.L., E.M. Coombs, G.P. Marking and D.B. Joley. 2004. Rush skeletonweed. In: E.M. Coombs, J.K. Clark, G.L. Piper, and A.F. Cofrancesco, Jr. (eds.), Biological Control of Invasive Plants in the United States. Oregon State University Press. pp. 293-303.

Smith, L., E. de Lillo, J.W. Amrine, Jr. 2010. Effectiveness of eriophyid mites for biological control of weedy plants and challenges for future research. Experimental and Applied Acarology 51: 115-149.

Supkoff, D.M., D.B. Joley, and J.J. Marois. 1988. Effect of introduced biological control organisms on the density of Chondrilla juncea in California. Journal of Applied Ecology 25:1089-1095.

Contributing Authors

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

Dr. Lincoln Smith, Research Entomologist, USDA