Photos

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Photo by Bob Richard, USDA-APHIS-PPQ.

Life cycle:
Over Wintering: The root-boring beetle overwinter as larvae in the root of the knapweed plant. (2)
Egg stage: Eggs are placed between the appressed petioles of diffuse knapweed rosette leaves during July and August. The flat eggs are white when deposited, but turn bluish-gray after four or five days. (2)
Larval stage: Second-instar larvae enter the petiole and mine down to the taproot where they overwinter. Feeding starts again in the spring. The larvae are long, whitish, with the front part of the body wider than the head. (2)
Pupil stage: Pupation occurs within a gall in the upper part of the root and lasts from late May or early June until mid-July to mid September. The length of the pupil period is about 15 to 21 days. (2)
Adult stage: They bite a hole in the gall wall and leave the root.  The new adults feed on diffuse knapweed leaves for two to three days before mating.  Egg laying begins in ten to twelve days after emergence. (1) The peak adult emergence occurs in July which coincides with the beginning of the flowering period of their host. Adults are about 8 mm to 10 mm long, somewhat flattened, and metallic, dark reddish brown. (2)

Damage to the host: Small knapweed rosettes that have been attacked often die by the end of the next  season. After the beetle population has built up for five to six years, the knapweed population may collapse with only scattered knapweed plants remaining. (1)

Impact on the Host:  Feeding on the leaves by the larvae depletes root carbohydrate reserves and stops rosette growth. (2)

Habitats: warm, dry areas. (1)

Release: It has been released in Arizona, Colorado, Idaho, Montana, Nebraska, Nevada, Oregon, South Dakota, Utah, Washington, and Wyoming. (1)

Redistribution: Adults can be collected with a sweep net during mid-July in the early evening. (2)

(2) Rees, Norman, et. al., Ed., Biological Control of Weeds in the West, Western Society of Weeds Science, in Cooperation With USDA ARS, Mt Department of Ag, and MT State Univ., Color World Printers, Bozeman Mt, Feb., 1996.