Larinus planus (F.)
Attacks: Bull thistle, Canada thistle, Nodding thistle, Plumeless thistle
Agent Type: Seed feeding beetle (weevil)
Origin: This native European insect found its way to North America before 1968.L. planus releases in BC descend from adventive populations found in 1988 in the Fraser Valley.
Adult: Larinus planus adult Weevils are oval shaped, 5-10mm long. Wing covers are slightly pock-marked with greyish-white tufts of hair. Their rostrum (nose) is long and narrow. Adults emerge from leaf litter in mid-June before Canada thistle has set bud and begin feeding in 2-3 days. Mating begins when temperatures reach 22°C, which occurs with bud formation, usually 14-26 days after they have emerged. Females seek out flowers for prime floral buds, choosing male or female flowers; acceptable bud sizes range from 5-7mm, ideal is 6mm; when buds enlarge over 7mm they are past suitability. Specific size is important for larvae development and coincides with the onset of rapid bud growth. A female chews cavities into unopened floral buds, deposits a single egg in each, and seals them with fecal material. A summer brood of adults emerge from buds in August and September, which feeds for a short time and prepare to overwinter. Adults disperse by walking or taking to flight.
Egg: The black "pin-prick" size oviposition point turns brown and an obvious distorted dimple can be observed. Eggs incubate over four days.
Larva/Pupa: Larinus planus larva The larvae have segmented white bodies with brown heads and typically are 'C' shaped. Upon hatching the larvae feed on the entire seedhead contents, consuming developing reproductive plant parts. If multiple eggs are deposited in a bud, only one larva will survive. In mid-summer pupation occurs inside a loose cocoon made from pappus and chewed bud material. New adults emerge through the top of the floral buds in August and September.
Overwintering: The summer adults overwinter in plant litter and debris near the plants' bases.
Location & Efficacy: A single larva consumes floral bud contents, destroying most or all of the seeds. Large Larinus planus populations can reduce seed production by 95%, greater than other seedfeeders. Seed reduction is most important on plumeless thistle, a biennial that reproduces strictly by seed. Adult feeding can be quite impressive, but provides less control than the larvae. Sites require a long time to develop populations large enough to show results.
Habitat: L. planus is capable of inhabiting wide geographic areas where Canada thistle exists. It prefers the dryer range of the invasive plant's habitat, however it adapts and slowly increases at moist sites. It prefers open sunny sites with coarse or well drained soils. Shaded locations or sites that flood or acquire water pooling are not ideal. Areas that are mowed or receive irrigation discourage establishment. It prefers Canada thistle over other thistles.
Native distribution in southwest England and throughout Europe; also commonly found in Morocco, Asia Minor, Caucasus Mountains, and central Asia. It is absent in central and northern Scandinavia.
In BC, L. planus has been released into the Bunchgrass, Boreal white and black spruce, Coastal Douglas-fir, Coastal western hemlock, Engelmann spruce-subalpine fir, Interior cedar-hemlock, Interior Douglas-fir, Montane spruce, Ponderosa pine and Sub-boreal spruce BEC zones. It has established and/or dispersed in all these zones except the Boreal white and black spruce zone.
BC History: In 1988 L. planus was discovered in Burnaby. As a result of these findings, screening started and was completed in Alberta. Approval occurred in 1988 and distribution began in 1989 into the southern interior. Assisted redistribution is still ongoing.
Field Results: L. planus has been found browsing on bull thistle and nodding thistle, however it is not known if full development occurs on these plants. It has been slow to accept BC's northern climates. L. planus does not drop away and feign death as easy as most weevils. This may contribute to predation. Additionally, Canada thistle is highly attractive to black aphids and ants, which have been seen attacking L. planus adults.
Handling/ Collecting: Aspirate adults from plants in June. Release into new locations which do not have a high ant population. Sites need to be exempt from activities that damage or remove flower buds.
Notes: Two generations of adults are present at one time, the spring-emerged mating adults and the August summer brood.
Field Guide: Operational Field Guide - Larinus planus