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Terellia virens
Diptera: Tephritidae

by R.F. Lang, USDA-APHIS-PPQ, Bozeman Biocontrol Facility, Forestry Sciences Laboratory, Montana State University, Bozeman, MT 59717- 0278.

The seedhead fly, Terellia virens, a native of Europe, was approved for release in 1992 and has been released in California, Colorado, Idaho, Minnesota, Montana (recovered), Nebraska, Oregon (established), Washington, and Wyoming (recovered).

Appearance

T. virens is a small (3-5 mm), clear winged, greenish-brown fly. It has bright green eyes.

Habitat

Spotted knapweed ( Centaurea maculosa) is a weed species that is found throughout the northern tier of states and as far south as Nebraska and Virginia. This highly competitive weed favors and establishes quickly on disturbed sites and overgrazed rangeland as well as established grassland communities out competing the native vegetation. The release of T. virens is part of a program to introduce a complex of spotted knapweed enemies to help control this weed.

Pest Attacked

T. virens has been host tested against fifty-seven plant species. The test results confirmed that T. virens has a limited host range and a preference for species in the Centaurea subgenus Acrolophus. Spotted knapweed ( C. maculosa ) is T. virens primary host. There has been very limited attack on diffuse knapweed ( C. diffusa)

Life Cycle

The adult T. virens begin to emerge in late May. Weather conditions determine if there will be two generations in a season. If so, most of the second generation will overwinter in the seedheads of the mature plants as prepupal larvae. Female T. virens lays her eggs in young, opening flowers. Each female lays and average of 80 eggs which hatch within three to five days after oviposition. The larval development to pupation takes about fourteen days. The larvae spend their first two instars inside a single seed. Later instars feed on mature and maturing seed. A seedhead may be infested by one to seven T. virens larvae. The adult flies are long lived and may live for forty-eight plus days.

Pesticide Susceptibility

Not yet known.

Commercial Availability

T. virens is not yet available from public or commercial sources.

Effectiveness

T. virens larvae cause considerable feeding damage on parts of the seed. A test for germination viability of 500 seeds was conducted. The control, no T. virens infestation, had 77.6% germination or 388 viable seeds compared to the T. virens infested seeds with 6.6% germination or 33 viable seeds. T. virens is released as a part of a complex of biocontrol agents for spotted knapweed will help control the weed.

Conservation

Release sites should have the potential of being free from disturbance such as development and pesticide use for at least ten years.

References

Groppe, K. and K. Marquardt. 1989. Terellia virens (Loew) (Diptera: Tephritidae), a suitable candidate for the biological control of diffuse and spotted knapweed in North America. Intl. Inst. of Biol. Control, DelÈmont, Switzerland Final Report. 30 p.

MÅller-SchÑrer, H., C. Lehr, M. Klein, and K. Marquardt. 1991. Gel-electrophoretic description of European populations of Terellia virens (Loew) (Diptera, Tephritidae): implications for its use as an agent for the biological control of Centaurea spp. (Asteraceae) in North America. Experientia 47: 859-864.

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