Terellia virens
<<back to spotted knapweed
Common Name
Green clearwing fly
Weed Impacted
This fly attacks both spotted and diffuse knapweed, but generally prefers spotted knapweed. Larvae inhibit plant reproduction by feeding on seeds within flower heads.
Insect Description
Adults are light yellow-greenish flies, 4 to 5 mm (approx 1/8 to 3/16 in) long, with clear wings and bright green eyes. Mature larvae are yellow-brown, maggot-like, and found within flower heads.
Life Cycle
In the fly’s native range (Europe), adults emerge in mid-May through August and live for up to a month or more. Females lay eggs one week after emergence. Eggs are laid in newly opened flowers and hatch in three to five days. Larvae feed on seeds and other tissue within the flower heads, where they also overwinter and pupate the following spring.
Field Identification
Several insect agents have been released for control of knapweeds. In the field, T. virens can be distinguished from the other knapweed flies by the presence of clear wings. All other knapweed flies have banded or patterned wings.
Availability
T. virens is currently only available in limited numbers in Oregon. Infested seed heads are collected in the fall. To inquire about availability and to request this biocontrol agent, please download, complete and submit this form.
Comments
This agent is most appropriate on cool, wet sites. Widespread establishment of the fly has been difficult due to competition with other seed head agents.
Using the Agent: Integrated Weed Management (IWM)
Several seed head agents have been released for control of knapweed. Because competition among agents may weaken any potential effects, releases of T. virens should be made only where other seed head agents are not abundant. Releases should be made only at sites where knapweed infestations are large and immediate eradication of the weed is not the primary objective.
Smaller, satellite knapweed populations can either be hand-pulled or sprayed. Consult with your county weed office, Extension office or an IWM practitioner to determine which herbicide treatments are safe and appropriate. Herbicide applications on larger infestations should only be made after insect agents have definitively established, and then should be timed according to the life cycles of both the plant and the insect.
Mowing will not control knapweed, and mowing while plants are flowering interferes with the establishment and maintenance of strong agent populations. Repeated grazing in the spring by sheep and goats (not horses) can weaken plants; but like mowing, grazing while plants are flowering can be detrimental to seed head insects.
Prescribed burning can destroy aboveground plant parts, promote germination of existing seeds, and facilitate earlier treatment of developing plants. However, it also increases the susceptibility of sites to new invasions by knapweed or other weeds.
Any management of knapweed should involve revegetation with native or other desirable plants, especially grasses. Revegetation promotes competition between plants, which stresses weeds and allows 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.
Seastedt, T. R., D. G. Knochel, M. Garmoe, and S. A. Shosky. 2007. Interactions and effects of multiple biological control insects on diffuse and spotted knapweed in the Front Range of Colorado. Biological Control 42(3):345-354.
Photo Credit
Terellia virens adult - USDA archive photo, bugwood.org (insectimages.org), http://www.insectimages.org/images/768x512/1350081.jpg
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