Chaetorellia australis 
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Common Name
Yellow starthistle peacock fly
Weeds Impacted
This fly attacks yellow starthistle and bachelor’s button (Centaurea cyanus) a related exotic species. Larval feeding within seed heads can reduce seed production by up to 90%.
Insect Description/Field Identification
Adults are small, straw-colored flies, 3-4 mm (approx. 1/4 in) long, with black spots on the back and dark bands on the wings. Larvae are whitish and maggot-like, lacking a head capsule, and found within the seed heads.
Life Cycle
C. australis typically completes two or more generations in a year. Adults begin emerging from seed heads in early April. Females lay eggs under the bracts of flower head buds. Hatched larvae feed within seed heads on developing seeds, and remain in the damaged seed head to pupate.
Availability
This agent is readily available in Washington, Idaho, Oregon, and California. Seed heads containing larvae are typically collected in late winter. However, because its impacts are usually secondary relative to E. villosus, and because it is likely already present at most starthistle sites, redistribution may be unnecessary.
Comments
C. australis does best at lower, non-coastal elevations and latitudes. In some locations, it may not establish where yellow starthistle occurs without bachelor’s button.
Using the Agent: Integrated Weed Management (IWM)
New releases of C. australis should only be made where other seed head agents (Eustenopus villosus) have not established. Releases should be made where yellow starthistle infestations are large and immediate eradication is not the primary objective.
Individual plants and smaller, satellite populations can either be hand-pulled or sprayed, although studies have shown that yellow starthistle can develop resistance to some herbicides. Consult with your county weed office, Extension office or an IWM practitioner to determine which herbicide treatments are safe and appropriate.
Any management of yellow starthistle should involve revegetation with native or other desirable plants, especially grasses. Competition between plants stresses weeds and can allow for more effective treatment. To learn more about revegetation and IWM, click here.
References
Coombs, E. M., J. K. Clark, G. L. Piper, and A. F. Cofrancesco, Jr. 2004. Biological Control of Invasive Plants in the United States. Western Society of Weed Science, Oregon State Univ. Press, Corvallis.
Gibbs, L.A., M.W. Murray, T.M. Sterling, and N.K. Lownds. 1995. Comparison of picloram-resistant and -susceptible yellow starthistle in a replacement series. Proceedings, Western Society of Weed Science 48:26.
Sterling, T.M., N. K. Lownds, E.P. Fuerst, T.S. Prather, and R.H. Callihan. 1991. Potential mechanism of picloram resistance in yellow starthistle (Centaurea solstitalis L.). Weed Science Society of America Abstracts 31:80.
Photo Credits
Chaetorellia australis adult - Neal Spencer, USDA Agricultural Research Service, bugwood.org http://www.insectimages.org/images/768x512/0022056.jpg
Chaetorellia australis larva - Gary L. Piper, Washington State University, bugwood.org http://www.insectimages.org/images/768x512/1350032.jpg
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