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Larinus minutus
Coleoptera: Curculionidae

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

Larinus minutus (seedhead weevil), a native weevil of Europe, was cleared and first released in the United States in 1991. The weevil has been released in Arizona, California, Colorado, Idaho, Minnesota, Montana (established), Nebraska, Oregon (established), South Dakota, Utah, Washington (established), and Wyoming (established) as a part of a biological control program to control spotted and diffuse knapweed (Centaurea maculosa Lam. and C. diffusa Lam.).

Appearance

L. minutus is a brown-grey weevil with a very large, bulbous snout. L. minutus measures 4-5 mm in length. They are strong fliers and disperse throughout the entire knapweed patch in several years. When the adults first emerge from the knapweed seedhead they are light grey with some yellow fuzz on their bodies.

Habitat

Spotted and diffuse knapweed are weed species that can be found throughout the northern tier of states and as far south as Nebraska and Virginia. These highly competitive weed species favor and establish quickly on disturbed sites and overgrazed rangeland. Both weeds will invade well established grassland communities and out compete the native vegetation. The release of L. minutus is part of a program to introduce a complex of spotted and diffuse knapweed enemies to help control these weeds.

Pests Attacked

L. minutus is host specific and limited to a few species in the genus Acrolophus and Calcitrapa. L. minutus was host tested with plants in the subgenera that are native, endangered or of economic importance before being cleared for release in the United States. L. minutus attacks both spotted and diffuse knapweed with a slight preference for diffuse knapweed.

Life Cycle

Adult L. minutus weevils emerge in late spring from the ground litter where they have been hibernating. The female weevil must feed on the flowers of the knapweed for ovary development. Therefore egg laying begins after the knapweed has started to bloom.. The eggs hatch within three days and the larvae begin feeding on the flower and migrated to the seeds and commence feeding. The seeds are consumed and when development of the larvae is complete the larva pupates and emerges for the seedhead as a new adult. Development from egg to adult takes about four weeks. The newly emerged adults feed on the plant foliage and eventually go into the soil and ground litter to hibernate for the winter.

Pesticide Susceptibility

Not yet known

Commercial availability

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

Effectiveness

L. minutus larvae destroy up to one hundred percent of the seed in an infested knapweed seedhead. This weevil along with other seed feeders will further reduce the seed that spotted and diffuse knapweed are dependent on for reproduction, dispersal, and survival.

Conservation

Sites that are chosen for L. minutus release should be considered for long term availability and should be left undisturbed by development or pesticide use for at least ten years. The release site should be dry with some bare ground and knapweed plants should be one to two feet apart. Frequently this will be the outer edges of the knapweed infestation.

References

Groppe, K. 1990. Larinus minutus Gyll. (Coleoptera: Curculionidae), a suitable candidate for the biological control of diffuse and spotted knapweed in North America. Intl. Inst. of Biol. Control, DelÇmont, Switzerland Final Report. 30 p.

Groppe, K., R. Sobhian, and J. Kashefi. 1990. A field experiment to determine host specificity of Larinus curtus Hochhut (Col., Curculionidae) and Urophora sirunaseva Hg. (Dipt., Tephritidae), candidates for the biological control of Centaurea solstitialis L. (Asteraceae), and Larinus minutus Gyllenhal, a candidate for biological control of C. maculosa Lam. and C. diffusa Lam. J. Appl. Ecol. 110: 300-306.

Jordan, K. 1995. Host specificity of Larinus minutus Gyll. (Col., Curculionidae), an agent introduced for the biological control of diffuse and spotted knapweed in North America. J. Appl. Entomol. 119: 689-693.

Lang, R. F., J.M. Story, and G.L. Piper. 1996. Establishment of Larinus minutus Gyllenhal (Coleoptera: Curculionidae) for biological control of diffuse and spotted knapweeds in the western United States. Pan-Pacific Entomol. 72: 209-212.

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