Biological Control Agent: Larinus obtusus Gy .
Type of Agent: Seed feeding beetle (weevil)
Status
Secondary
Description and Biology
Adult:
Weevils are 4.5 – 6.5 mm long, mottled brown-black coloured 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 to 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 into the center of the bud, damaging 3
- 4 florets and lay a single egg which they then cover with a protective secretion. I f the flower is large enough to support more larvae, the females will lay up to
five eggs into spotted knapweed. An average of 130 eggs is laid over
seven weeks, usually at a rate of seven eggs per day. Their life span is 97 days for males and 58 days for females. Although males 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.
Egg:
Eggs are yellow, oval, and measure 1.28 x 0.84 mm. They require a minimum temperature of 13°C for development. At 32°C they hatch in 1.5 days and at 25°C they take 2.5 days.
Larva and pupa:
Larvae are small white, slightly yellow and ‘C’ shaped, with light brown heads. There are three larval instars that complete develop within the flower. Head capsule measurements is used to determine each instar. Newly hatched larvae feed on pappus. At this early stage, multiple larvae will complete against each other, killing others until a number is achieved that the bud can support. Larvae mature in 17 days. They pupate in an upright hard cocoon, 5 – 8 mm x 8 – 9 mm. Pupation lasts
nine days and new adults emerge by chewing through the pupal case, moving upward and out and leaving behind a dark cavity. They browse on plant foliage until hibernation.
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.
Predicted and native habitat
Larinus obtusus shares similar habitat requirements as L. minutus, attacking several knapweeds, but, shows a preference for spotted knapweed. It favours moister sites with cooler temperatures than those tolerated by L. minutus. It establishes on south and west slopes with well drained coarse soils, often near water. Excess competing vegetation may discourage establishment.
Its native geographic distribution is from Central Europe east to the Caucasus mountain region.
British Columbia Experiences
L. obtusus was first introduced to open field releases in 1992 and 1993 in the Southern Interior. While the sites in the Kootenays established, others did not. In 1999, 14,800 adults were field collected from the Nelson area and redistributed. Of the 33 new sites, 29 established within a single year. It has been released into the Bunchgrass, Coastal Douglas-fir, Coastal
western hemlock, Interior Douglas-fir, Montane spruce, and Ponderosa
pine biogeoclimatic zones. Establishment has been confirmed in all zones except Montane
spruce zone. L. obtusus has established on meadow knapweed and
short-fringed knapweed in BC. It does not establish on diffuse knapweed, which grows in an unfavourable habitat.
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. obtusus can 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.
It is the last of the seed feeding agents to oviposit and competing agents can limit its success.
References
Groppe, K. 1992. Larinus obtusus Gyll. (Col.: Curculionide). A candidate for biological control of diffuse and spotted knapweed. C.A.B. Internat. Inst. Biol. Contr. Final Report. 46 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.
Rees, N. E., Quimbly, Jr., P. C., G. L. Piper, E. M. Coombs, C. E. Turner, N. R. Spencer, L. V. Knutson (editors). 1996. Biological control of weeds in the west.
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