Biological Control Agent: Rhinusa antirrhini (Paykull)
Type of Agent: Seed head feeding weevil
Description and Biology
Note: Unless noted otherwise, the information provided represents both strains of
Adults are oval, black bodied and 5 mm long when on Dalmatian toadflax and 3 mm long when on narrow-leaved Dalmatian toadflax. The rostrum (nose) is curved and pointed and from the side it appears tapered. All but the tip of the rostrum is covered with pubescence. Their bodies have fine yellow-brown hairs arranged in rows. Adults appear in May when plants are 20 – 30 cm tall. They feed first on tender new growth which initiates branching lateral growth, then they move to flowers to feed on pollen and young seeds. Several adults can be present in a single bloom. R. antirrhini normally lay eggs from June through August, often having to delay oviposition until Brachypterolus pulicarius damage has subsided. When the flowers have enlarged to just about as large as they will become, the females will prepare oviposition sites by chewing into the green pods. A single egg is inserted into each cavity, but, up to
eight may be laid into each pod, which are covered with a yellow secretion. Within one week, a wart-like or nipple-like protrusion can be seen just above the oviposition point. At the same time, the egg is lifted into the protrusion, which prevents it from becoming damaged while also placing the egg deep into the developing seeds. The seeds nearest the egg swell abnormally, 8 – 10 times larger, and become pale coloured. Females lay an average 54 eggs, but, they range from 10 to 132 eggs and will continue to lay as long as suitable flowers are available, even though eggs or new larvae may not survive the winter. New adults from the spring mating will emerge in August and September but not mate or oviposit, instead they overwinter. Adults that emerged in spring and early summer will live until September.
The eggs are oval but flattened, and 0.49 x 0.27 mmin length. Incubation takes 12 – 17 days.
Larva and pupa:
Full grown larvae are 0.54 mm long and have a clear dark brown head. Typical to weevils, their distinct ‘C’ shape can be easily identified in pods when broke open. The first larvae instars feed on the pale enlarged seeds. Several larvae can occupy a single seed pod chamber. The final instar may feed on regular developing seeds. Pupation takes 10 – 15 days, occurring within the seedpod where mature larvae build an oval cell. R. antirrhini can be identified by two pointed horns at the top of their thorax. New adults emerge through the upper opening of dried seed capsules.
Adults overwinter within soil duff. Sometimes they will also spend winters above ground inside the dried seed capsules.
Location and effectiveness of attack
Larvae feed on seeds, completely destroying all the seeds in the capsule that they occupy. Adults feed on foliage which can add to plant stress and reduces vigour. Adult pollen and flower feeding also contribute to decreased seed production. On yellow toadflax R. antirrhini can reduce seed production by 20-25%.
Predicted and native habitat
There are two strains of R. antirrhini; each showing a preference for either Dalmatian or yellow toadflax varieties. The strain found on yellow toadflax is believed to have been unintentionally introduced.
The first North American published record indicates it was present in Massachusetts in 1909, (earliest Canadian specimens were found in 1917 in Quebec).
Dalmatian toadflax and narrow-leaved Dalmatian toadflax strain
R. antirrhini habitat is limited to where summers are warm to hot and winters are mild. Adequate snow cover adds protection where winters may be harsh. It prefers open sunny locations and is common on south and west aspects. Dense stands growing in well drained soils are ideal.
Yellow toadflax strain
Commonly found and establishes easily on host plants in a variety of climates.
The native distribution of R. antirrhini is from north Africa (Algeria) to the northern limits of yellow toadflax, into the Baltic countries and is frequently found in the cool moist habitats of Europe.
British Columbia Experiences
Dalmatian toadflax strain
The first R. antirrhini release in BC was made in 1996 near Kamloops onto Dalmatian toadflax growing in a bunchgrass habitat. Early monitoring indicated no establishment and soon after access to the site was closed preventing further follow up. Subsequent releases have since been made throughout BC. By 2007, 98% of all treatment sites have established. In 2005, weevils were found and following positive identification of R. antirrhini, dispersal monitoring was initiated around this early release. Results showed that R. antirrhini had established near Kamloops and was subsequently found great distances from the original release point. As well, the weevil has dispersed significantly in the North Thompson. It is probable it is widely spread in other parts of the Southern Interior, including Princeton, the Okanagan, and the Kootenays.
R. antirrhini appear to be easier to tease from flowers than
R. neta. R. antirrhini establishes easily in the Thompson – Okanagan, and Kootenay climates. Established sites and dispersal locations occur in the Bunchgrass, Interior cedar-hemlock, Interior Douglas-fir, Montane spruce and Ponderosa Pine biogeoclimatic zones.
Mecinus janthinus attack creates delayed flowering and may cause R. antirrhini to disperse and/or oviposit later in the season. When competition is heavy among the bioagents, the few flowers blooming in late summer invites adults to congregate in large numbers. On a single plant, R. antirrhini, R. neta, and M. janthinus were found in August. In the rearing tents, adults have been observed mating and ovipositing well into late August. It is not known what success the late oviposited eggs and hatching larvae have for overwintering. Seed pods attacked by Rhinusa spp. can be detected by a pale tan coloured, grainy textured substance that can be seen from the open end of the pod. When these are broke open, one or more larvae, pupae or developed adults can be found unless the weevils
have already emerged. New adults ready to emerge can often be seen with their heads or posteriors visible through the opening as they will have moved the grainy, tan particles aside for easy emergence. As well, adults may chew a hole through the pod wall to emerge as an alternative to the natural opening at the top.
Yellow toadflax strain
No releases have been made in BC. However, the insect is commonly found on yellow toadflax throughout the Kamloops area. It is suspected to thrive throughout the Southern Interior wherever the host plant establishes. Larvae were found in seedpods in early May at cool, semi-shaded locations in the Interior Douglas-fir zone. Adults were found in the Southern Interior during yellow toadflax bloom from late May through August.
Collection for redistribution
Plants can be swept for adults from June through September. Aspirating is most effective and less damaging to the plants.
When existing on the same plants as R. neta in their natural European habitat, R. antirrhini is 50% smaller. When combined with B. pulicarius, seed reduction can be 85 – 95%.
Formerly known as Gymnetron antirrhini and Gymnaetron antirrhini.
Harris, P. and A. Gassmann. 2003. Classical biological control of weeds. Gov. of Can., Agriculture and Agri-Food Canada. May 20, 2003. http://res2.agr.ca/lethbridge/weedbio/agents/agymnet_e.htm
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.
Smith, J. Morris. 1959. Notes on insects, especially Gymnaetron spp. (Coleoptera: Curculionidae), associated with toadflax, Linaria vulgaris Mill. (Scrophulariaceae), in North America. Can. Entomol. Vol. XCI, No. 2: 116-121.