Biological Control Agent: Larinus minutus Gyll.

Invasive Plant Species Attacked: Spotted knapweed (Centaurea biebersteinii), diffuse knapweed (C. diffusa) and meadow knapweed (C. debeuxii)

Type of Agent: Seed feeding beetle (weevil)

Status

Description and Biology

Adult:

Weevils are 5 – 10 mm long, mottled rusty brown, with a line of short yellowish hairs on their wing covers. Their rostrum (nose) is short, bent and blunt. They are strong fliers and readily take flight on hot days. Overwintered adults emerge in late spring before the plants have set bud and feed on foliage, stems and seedlings. Females require sufficient feeding on knapweed to develop mature ovaries. Mating occurs from late morning through mid afternoon, beginning from the onset of the flowering period (usually four weeks after emergence). Females oviposit into fresh flowers that have just opened. They chew holes in the center of the bud, damaging 3 - 4 florets, and lay a single egg which they cover with a protective secretion. If the flower is large enough to support more larvae, the females will lay up to three eggs in diffuse and five eggs into spotted. An average of 130 eggs is laid over seven weeks, usually depositing seven eggs each day. Their life span is 97 days for males and 58 days for females. Although males appear to have a longer life span, more females are found in the field during knapweed flowering. Some adults will hibernate a second year, but it is not known if they will reproduce the following summer. In part of their native range in Greece, they avoid extreme hot mid day temperatures, seeking shade and moving onto lower plant parts.

Egg:

Eggs are yellow, oval shaped and measure 1.28 x 0.84 mm. They require a minimum 13 °C for development. At 32°C they hatch in 1.5 days and at 25°C they will take 2.5 days.

Larva and pupa:

Larvae are small, white, slightly yellow and ‘C’ shaped, with light brown heads. There are three instars that complete development within the flower. Head capsule measurements are used to determine each instar. Newly hatched larvae feed on pappus. At this early stage, multiple larvae will compete against each other, killing off others until a number is achieved that the bud can support. The larvae mature in 17 days. They pupate in spotted knapweed in an upright, hard cocoon, 5 – 8 mm x 8 – 9 mm. In diffuse knapweed the cocoon is lighter weight or absent. Pupation lasts nine days and new adults emerge by chewing through the pupal case, moving upward and out and leaving behind a dark cavity.

Overwintering stage:

Adults overwinter in cracks and crevices in the soil and in plant litter.

Location and effectiveness of attack

Larvae feeding in flowers cause significant decreased seed production. In trials L. minutus is the main seed feeder and creates the most impact.

Predicted and native habitat

Typically Larinus minutus prefers dryer sites with hotter temperatures than those tolerated by L. obtusus. It favours dense knapweed stands with little plant competition. It requires well drained, coarse soils; avoiding compacted sites. Open areas with south aspects at elevations between 300 and 800 have proven successful. Larinus spp. is the last of the seed feeding biocontrol agents to oviposit and competing agents can limit its success.

Its native geographical distribution is from Bulgaria, Greece, Israel, Rumania, Caucasus Mountains, Kazakhstan, and southern western parts of the former USSR.

British Columbia Experiences

L. minutus was first introduced in 1991. The shipment was divided into several Southern Interior field releases and the rearing facility in Kamloops. That same year, a second generation was collected from the propagation tent and released near Castlegar. In 1994, field sites began to produce enough adults to become collection sources. From 1994 to 1998, there were 78,766 adults collected from field sites for redistribution. L. minutus is established and dispersed throughout knapweed infestations growing in Bunchgrass, Interior cedar-hemlock, Interior Douglas-fir, and Ponderosa pine biogeoclimatic zones. It is also detected on the fringes of Coastal Douglas-fir and Coastal western hemlock zones.

Collection for redistribution

Sweeping for adults during peak emergence, from June to late July, on hot days is rapid and effective. Use heavy sweepnet bags and aspirate clean collections as the adults climb towards the top opening. On hot bright days adults take flight quickly. When females are actively ovipositing, they will cling tenaciously to the plant. Care must be taken not to harm them when sweeping. It is best to collect L. minutus before this time so one can be assured the females transported to new sites will have eggs to establish a new population.

Notes

L. minutus can co-exist with Urophora affinis because the fly attacks the plants earlier and is shielded with a protective gall. It does however avoid using the same buds.

A variation in adult size is a direct result of competition for food during the larval development stage.

References

Groppe, K. 1990. Larinus minutus Gyll. (Coleoptera: Curculionidae), a suitable candidate for the biological control of diffuse and spotted knapweed in North America. C.A.B. Internat. Instit. Biol. Contr., European Station Rep. 31 p.

Harris, P. 2007. Classical biological control of weeds established biocontrol agent Larinus obtusus Gyll. soft – achene feeding weevil. Gov. of Can., Agriculture and Agri-Food Canada. February 7, 2007. http://res2.agr.ca/lethbridge/weedbio/agents/alarobt_e.htm

Mason, P. G. and J. T. Huber, (editors). 2002. Biological control programmes in Canada, 1981-2000. CAB International.

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. 1999. Operational field guide to the propagation and establishment of the bioagent Larinus minutus (Knapweed seedhead weevil). Min. For., For. Practices Br., Range Sect., Noxious Weed Control Program.