Cyphocleonus achates
(Coleoptera: Curculionidae)

by Ronald Lang, USDA-APHIS-PPQ, Forestry Sciences Lab, Montana State University, Bozeman, MT 59717-0278.

The root boring weevil, Cyphocleonus achates, was first released in the United States in 1987. Its release is part of a program to introduce a complex of spotted and diffuse knapweed enemies to help control these natives of Eastern Europe and Asia. In North America spotted and diffuse knapweed are efficient competitors and since being introduced around the turn of the century have spread throughout the northern tier of states and as far south as Nebraska and Virginia. They favor disturbed sites and overgrazed rangelands and cause a reduction of forage for cattle of up to 100% in infested areas. Spotted knapweed and diffuse knapweed also prosper on grasslands and riparian sites and displace native vegetation in these habitats.

Although knapweed can be controlled by extensive cultivation or herbicide application, its existence on stream banks and rough terrain and the typically expansive areas of infestation make these methods impractical and/or economically unfeasible. Therefore, an integrated pest management approach is being sought to help in controlling this weed.

C. achates has been released in Arizona, California, Colorado, Idaho, Minnesota, Montana, Nebraska, Nevada, Oregon, South Dakota, Utah, Washington, and Wyoming as part of a program to control spotted and diffuse knapweed. Populations are established in Colorado, Montana, and Wyoming, and individuals have been recovered in Oregon. It has also become established in Minnesota (N. Northrop, personal comm.).

Appearance

C. achates is a large, 20 mm long, brown-gray mottled weevil. Its markings and coloration blend with the ground and the mature seedhead on the knapweed plant. Its habit is to sit perfectly still and when disturbed to drop to the ground and play dead. C. achates does not fly.

Habitat

Spotted and diffuse knapweed-infested areas.

Pests Attacked

C. achates is considered host specific to spotted and diffuse knapweed. The host tests were conducted on seventy-one plant species, and development from larva to adult occurred on four of the seventy-one plant species: Centaurea maculosa (European and American bio-types), C. diffusa (European and American bio-types), C. micranthos (European), and C. vallesiaca (European), all of which belong to the subgenus Acrolophus.

Life Cycle

C. achates overwinters as larvae in the tap root of spotted and diffuse knapweed. Adults emerge from late July through late September. The adults feed on the knapweed leaves, preferring tender leaves in the center of the rosettes. The eggs are laid on the plant root just below the soil surface and hatch in ten to twelve days. The larvae burrow into the root and commence feeding on the root of the spotted and diffuse knapweed plant. Multiple larvae may be in the same root. The female weevil copulates many times during the egg laying season and lays an average of sixty-five eggs during her lifetime. C. achates produce one generation per year.

Pesticide susceptibility

Not yet known.

Commercial availability

In some states, C. achates adults may be obtained at no cost from state weed management agencies.

Effectiveness

Plants are stunted and survive one season after being infested with C. achates. Larvae destroy the interior of the tap root and expose it to bacterial and fungal invasion which helps kill the plant.

It is anticipated that this weevil will be an effective control agent in conjunction with other root boring and seed feeding insects. Spotted and diffuse knapweed patches are hard to find in Europe because they are kept under control with the native insects and diseases.

Conservation

Long-term availability is important in sites that are chosen for C. achates releases. It is wise to consider sites that will be undisturbed by development or pesticide use for at least ten years. The weevil needs high soil temperature to complete development and cannot establish in areas with a short growing season. Therefore this weevil should be released in areas or microclimates where the soil is partially exposed and absorbs heat. The weevils do best on a south facing slope that is somewhat barren with rocky exposure.

For general information about conservation of natural enemies, see Conservation in the Tutorial section on this site, or the Volume II, No. 1Feature Article on conservation in the Midwest Biological Control News Online.

References

Stinson, C. S. A., D. Schroeder, and K. Marquardt (1994) Investigations on Cyphocleonus achates (Fahr.) Col., Curculionidae, a potential biological control agent of spotted knapweed (Centaurea maculosa Lam.) and diffuse knapweed (C. diffusa Lam.) (Compositae) in North America. J. Appl. Ent. 117:35- 50.

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