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Larinus obtusus Coleoptera: Curculionidae The seedhead weevil, Larinus obtusus, a native of Europe, was approved and first released in 1993 in the United States. This weevil has been released in Idaho, Minnesota, Montana (established), Nebraska, Oregon, and Washington (established) as part of a program to control spotted and diffuse knapweed (Centaurea maculosa Lam. and C. diffusa Lam.).
Appearance L. obtusus is a grey-brown weevil (5 mm) with a prominent, bulbous snout. When the adults emerge from the seedhead they are a lighter grey and have yellow fuzz on their bodies. Older L. obtusus tend to be darker, sometimes appearing black.
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. obtusus is part of a program to introduce a complex of spotted and diffuse knapweed enemies to help control these weeds.
Pests Attacked L. obtusus was host tested against seventy-eight North American plant species including closely related, native, endangered, and economically important species. The weevil, after these tests, is considered to be host specific to Centaurea species in the subgenera Acrolophus and Jacea. Spotted knapweed belongs to Acrolophus and is one of the plants attacked by L. obtusus. In Europe the host plants for L. obtusus are C. phrygia (Romania) and C. jacea (Yugoslavia), with some attack on C. maculosa (spotted knapweed) in the area.
Life Cycle L. obtusus adults emerge from the soil and litter in late spring. The adults feed on the leaves and flowers of spotted knapweed. Mating and egg laying begin with the onset of the spotted knapweed flowering. The eggs are laid in the flower, and the larvae hatch and burrow into the flower head and begin feeding on the capitula. When the larvae mature they form a hard walled "cocoon" with pappus hairs, seeds and secretions. Mating and egg laying lasts from five to six weeks after the plant starts to flower. The newly-emerged adults feed heavily on the knapweed plants and hibernate in the soil and ground litter.
Pesticide susceptibility Not yet known
Commercial availability L. obtusus is not yet available from public or commercial sources.
Relative Effectiveness L. obtusus larvae destroy nearly one-hundred percent of the seeds. Any seeds that are not eaten will become a part of the "cocoon". More than one larva may develop in a seedhead.
Conservation When choosing a site it is important to select a site that will not be disturbed by development or pesticide use for a least ten years. L. obtusus prefers moist sites in contrast to the other seedhead weevils for knapweed which prefer and thrive in drier sites.
References Groppe, K. 1992. Larinus obtusus Gyll. (Col.: Curculionidae): a candidate for biological control of diffuse and spotted knapweed. Intl. Inst. of Biol. Control, DelÇmont, Switzerland Final Report. 46 p.
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