Biological Control Agent:  Larinus planus (F.)

Invasive Plant Species Attacked: Canada thistle (Cirsium arvense) and plumeless thistle (Carduus acanthoides). Adult foliar feeding on bull thistle (Cirsium vulgare) and nodding thistle (Cirsium nutans).

Type of agent: Seed feeding beetle (weevil)

Status

Secondary

Description and Biology

Adult:

Weevils are oval shaped, 5 – 10 mm 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^oC, 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 that fit specific criteria. Acceptable bud sizes range from 5 – 7 mm, ideal is 6 mm; when buds enlarge over 7 mm 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 and pupa:

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 stage:

The summer adults overwinter in plant litter and debris near the plants’ bases.

Location and effectiveness of attack

A single larva consumes floral bud contents, destroying most or all of the seeds. Large Larinus planus populations can reduce seed production by 95%, which is 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.

Predicted and native 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 feeds on several thistles, but it prefers Canada thistle over others.

The native distribution occurs in southwest England and throughout Europe is commonly found in Morocco, Asia Minor, Caucasus Mountains, and central Asia. It is absent in central and northern Scandinavia.

British Columbia Experiences

This European insect found its way to North America prior to 1968. In 1988 L. planus was discovered in Burnaby. As a result of these findings, screening commenced and was completed in Alberta. Approval occurred in 1988 and distribution began in 1989 into the Southern Interior. By 2006 it was established and dispersed into the Bunchgrass, Coastal Douglas-fir, Coastal western hemlock, Interior cedar-hemlock, Interior Douglas-fir, Montane spruce and Ponderosa pine biogeoclimatic zones. No recoveries have been found in the Mountain spruce or Boreal white and black spruce zones. It 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.

Collection for redistribution

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.

References

Harris, P. 2005. Classical biological control of weeds established biocontrol agent Larinus planus (F.) (=carlinae). Flower-head weevil. Gov. of Can., Agriculture and Agri-Food Canada. February 7, 2007. http://res2.agr.ca/lethbridge/weedbio/agents/alaripla_e.htm

Manitoba Agriculture. 1993. Canada thistle biocontrol insect profile Canada thistle seed head weevil. Biofacts. Govt. of Manitoba. 2 p.

McClay, A. S. 1989. Biology and host specificity of Larinus planus (F.) (Coleoptera: Curculionidae), a potential agent for Canada thistle, Cirsium arvense (L.) Scop. Govt. of Canada, Alberta Environ. Centre. 28 p.

MFR staff observations and comments

Powell, G. W., A. Sturko, B. Wikeem and P. Harris. 1994. Field guide to the biological control of weeds in British Columbia. B.C. Min. For. Res. Prog.

Province of British Columbia. 2001. Operational field guide to the propagation and establishment of the bioagent Larinus planus (Canada thistle seed-feeing weevil). Min. For., For. Practices Br., Range Sect., Noxious Weed Control Program. 47 p.