Biological Control Agent: Mecinus janthinus Germar.
Invasive Plant Species Attacked: Dalmatian toadflax (Linaria dalmatica), narrow-leaved Dalmatian toadflax (L. genistifolia spp. dalmatica) and yellow toadflax (L vulgaris).
Type of agent: Stem mining beetle (weevil)
Description and Biology
Dark black to somewhat blue coloured weevils are elongated in shape, 3.6 – 4.2 mm long, with distinct linear lines along their wing covers. The males have strongly toothed front femurs. They emerge in May and begin feeding on foliage, concentrating on tender terminal growth. Mating and egg laying begins immediately, continuing until mid July. Females chew cavities into stems and oviposit a single egg into each. Egg laying continues at a rate of 1.15 eggs/day for 2.5 months or more. Several eggs can be oviposited into each stem. Ideal stem calliper is 0.9 mm for successful development. Prostrate stems are not favoured. Finer stems can be oviposited into, but the larvae may fail to completely develop. Adults reared on yellow toadflax are usually smaller.
Eggs incubate for 6 – 7 days at 24°C (day) and 18°C (night). A visible callus forms at the oviposition site.
Larva and pupa:
Mecinus janthinus larvae are ‘C’ shaped, white larvae with pale brown head capsules. They require summer development temperatures to be 23 – 34 days at 24°C (day) and 18°C (night). Over 100 larvae may fully develop on a single stem, where each will mine about 1 – 3 cm length. Poor development occurs on the less nutritious prostrate stems. Pupae, 3.0 – 4.5 mm long, form in the stems in 30 – 40 days, changing from white to black over the pupation period. The following spring new adults chew the outer stem wall and exit through highly visible, distinct circular holes.
Adults overwinter in pupal cells in toadflax stems. Cold winter climates cause high mortality, however sufficient populations usually survive to continue attack the following year.
Location and effectiveness of attack
Larvae mine stems which significantly contributes to the plants’ inability maintain vigour and become productive. On Dalmatian toadflax, high populations of adult feeding prevents flowering and kills the upper plant stems. In these situations flowering is often delayed until the following year. Floral stalks can be reduced from several (six) to one. There is less physically noticeable impact on yellow toadflax stands despite when seed production was reduced by 68%. M. janthinus attack is further enhanced when plants become drought stressed.
Predicted and native habitat
M. janthinus prefers grassland or open forested areas with hot dry conditions. Sites need to have large stemmed plants to support larval development. The literature describes suitable areas may be limited to the southern interior of British Columbia or Alberta. Control is enhanced in locations where plants suffer summer drought stress.
Its geographic range is south and central Europe to south-western Russia, occurring between latitudes 40 and 52°N. Within these parameters it inhabits the sub-alpine and maritime areas associated with dry summer and moist summer sub-continental climates.
British Columbia Experiences
Two small populations were released in 1991. One was set into caged rearing tents and the other was placed into an open field site in Kamloops. Eventually the weevils were moved to open rearing plots. Subsequent weevils were collected from the rearing plots and later field sites developed as collection sources. In 1996, field collections began and in 1999, 27,294 adults were collected and redistributed to 129 sites. In 1999, hundreds of bundled stems containing larvae were removed from the rearing plots and distributed throughout the province. Weevils have been released and have established in the Bunchgrass, Coastal western hemlock, Interior cedar-hemlock, Interior Douglas-fir, and Ponderosa pine biogeoclimatic zones. M. janthinus known established range includes from the Canada/USA border north to Williams Lake. By 2005, M. janthinus populations were found widely dispersed, frequently seen on toadflax sites in the Southern Interior.
In large populations, aggressive attack from May through June leaves plants stunted, weak and unable to flower. Spring feeding causes lateral spur growths and subsequent flowers are smaller and less productive (smaller flowers and smaller seedpods). By late August, many Dalmatian toadflax leaves are covered with a black sooty residue. M. janthinus is found more abundantly on Dalmatian than on yellow toadflax. It has been found co-existing with Rhinusa antirrhini, R. neta, R. linariae, Calophasia lunula and Brachypterolus pulicarius.
Collection for redistribution
Aspirate adults from plants during May and June. Alternately clip larvae/pupae infected stems and redistribute them among plants at new sites.
M. janthinus is capable of existing on the same site with other seed, foliar and root feeding bioagents.
By 2001, M. janthinus had dispersed from Canada into Washington State.
Harris, P., R. DeClerck-Floate and A. McClay. 2005. Classical biological control of weeds established biocontrol agent Mecinus janthinus Germar. Stem boring weevil. Gov. of Can., Agric. and Agri-Food Canada. February 9, 2007. http://res2.agr.ca/lethbridge/weedbio/agents/amecinus_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.
Washington State. 2002. Annual report of Washington’s contributing projects to cooperative regional project W-1185 (W-185). 4 p