Insect Description
Larinus minutus and L. obtusus are very similar in appearance and biology. Eggs are elongate and white to yellow. They are found in pappus hairs. Larvae are approximately 8 mm long, white, c-shaped grubs with brown head capsules. They are found in seed heads where they also pupate. Pupae are slightly smaller (~6 mm) and white turning brown shortly before emergence. Adults are mottled brown with long snouts bent downward. L. minutus adults are slightly smaller (4-5 mm) than L. obstusus (5-7 mm).
Field Identification
It is very difficult to differentiate between the two species in the field, however, L. obtusus is slightly larger than L. minutus. It is also less hairy. Additionally, L. obtusus prefers moister sites often near water and on south and west facing slopes. L. minutus prefers hotter, drier sites. Both are found in areas where competing vegetation is sparse.
Life Cycle
Adult weevils are active from May through September. They emerge from the soil as immature adults and begin feeding on knapweed foliage, stems and flowers. Females lay eggs on pappus hairs of emerging flowers. Over 100 eggs may be laid by one weevil. Eggs mature in approximately three days. Larvae feed on seeds, florets and pappus hairs. They feed for approximately four weeks and go through three instars before forming a pupal chamber from plant material. Adults chew their way out of pupal chambers leaving behind an empty cylinder in the center of empty seed heads. Adults overwinter at the base of plants in soil. There is one generation per year.
Impacts
Both weevils reduce knapweed density through foliar and seed feeding hence reducing plant vigor and seed dispersal. While both weevils have been found on several knapweed species, L. obtusus reportedly prefers spotted knapweed and L. minutus prefers diffuse knapweed. Their impact on these species and others has varied regionally.
Availability
Both Larinus spp. are readily available in Montana. Due to their similarity, most releases will contain both species. If you are interested in obtaining these insects view the biocontrol vendor list for options.
Notes
Larinus spp are strong fliers and are found in most infestations of spotted knapweed in Montana. Landowners should monitor their knapweed infestations for Larinus establishment prior to purchasing or collecting insects to release.
Using the Agent
Adults are easily collected with a sweep net preferably before 50% bloom. After this females may have already laid a majority of their eggs. Adults can also be collected in the spring when they emerge from the soil at the base of plants. This weevil is best used in combination with other biocontrol agents and weed management strategies.
Smaller, isolated knapweed populations should be controlled by other means including hand-pulling and chemical controls. On large sites where weevils are established chemical controls should be timed with both plant and insect lifecycles in mind. Consult your local weed district or Extension Service for herbicide recommendations.
References
Winston, R., C.B. Randall, R. D. Clerck-Floate, A. McClay, J. Andreas, M. Schwarzlander. May 2014. Field Guide for the Biological Control of Weeds in the Northwest. http://www.ibiocontrol.org/westernweeds.pdf
Washington State University Integrated Weed Control Project. Biological Control Agents. Accessed January 18,2017. http://invasives.wsu.edu/biological/cyphocleonusachates.htm
NRCS Montana. “Biological Control of Diffuse Knapweed.” Accessed January 18, 2017. https://www.nrcs.usda.gov/wps/portal/nrcs/detail/mt/home/?cid=nrcs144p2_056780.