Biological Control Agent: Rhinusa neta Germar
Invasive Plant Species Attacked: Dalmatian toadflax (Linaria dalmatica), yellow toadflax (L. vulgaris) and narrow-leaved Dalmatian toadflax (L. gentistifolia spp. dalmatica)
Type of Agent: Seed head feeding weevil
Description and Biology
Rhinusa neta colouring is quite variable, from ash-grey to olive brown. Their oval, convex bodies are more rounded than other Rhinusa, and are usually significantly larger. Their rostrums (noses) are rigid, straight and have a blunt tip. Their bodies are covered with fine yellow-brown hairs arranged in rows. Adults appear in May when plants are 20 – 30 cm tall. At first, they feed on tender new growth which initiates branching lateral growth and they later move into flowers to feed on pollen and young seeds. Several adults can be present in a single blossom. Females normally lay eggs from June through August and sometimes need to delay oviposition until Brachypterolus pulicarius feeding damage has subsided. When flowers are near peak bloom, females chew round holes into the green seed capsules and singly deposit up to eight eggs, covering them with a yellow secretion. The base of the egg, pointed towards the oviposition hole, causes the site to become black and forms a barrier for the other end of the egg which is inflated. The barrier pushes the egg deep into the developing seeds. Seeds nearest the egg expand 8 – 10 times their normal size, becoming pale coloured. Each female averages 45 eggs, but, can range from 21 to 66. They will continue to lay eggs as long as suitable flowers are available, even though eggs or larvae may not survive the winter. New adults resulting from spring mating emerge in August and September but do not mate or oviposit, instead they overwinter. Adults that emerged in the spring or early summer live until September.
The eggs are oval and flattened, 1.23 x 0.67 mm when first laid, swelling over two or three days to 1.43 x 0.83 mm. Incubation takes 12 days.
Larva and pupa:
Full grown larvae are 0.63 mm long and have a vague light brown pattern. Typical to weevils, their distinct ‘C’ shape can be easily identified in pods when opened. There are three larvae instars. The first instars feed on the enlarged pale seeds. Several larvae can occupy a single seed pod chamber. The final instar may feed on regular developing seeds. Pupation occurs within the seedpod where mature larvae build an oval cell. They can be identified by two blunt horns at the top of their thorax. Five weeks after hatching from eggs, new adults emerge through the upper opening of dried seed capsules.
Adults overwinter in plant and soil duff. Sometimes they will also spend winters above ground inside the dried seed capsules.
Location and effectiveness of attack
Larvae feed on seed, completely destroying all the seeds in the capsule that they occupy. Adults feed on foliage which may add to plant stress and reduce vigour. Adult pollen and flower feeding also contribute to a decrease in seed production.
Predicted and native habitat
R. neta is native to Europe and was unintentionally brought to North America. The first North American published record indicates it was present in the eastern seaboard states in 1937, and the Pacific Northwest, including BC, in 1954. In Canada, it is common on invasive toadflax varieties in varying habitats and climates, including the prairies.
Its native distribution is south and central Europe, through the Mediterranean to Iran but is absent in Denmark and Sweden. It is most common in locations with hot dry summers.
British Columbia Experiences
No releases were made in BC, however, the agent found its way here and has freely dispersed adding to the control of toadflax. It has been found throughout the Southern Interior in the Interior Douglas-fir, Montane Spruce, Bunchgrass and Ponderosa Pine biogeoclimatic zones. It establishes on Dalmatian and yellow toadflax varieties along side Mecinus janthinus, R. antirrhini, B. pulicarius and R. linariae. R. neta are harder to be teased or coax out of flowers than R. antirrhini. Initially thought to have a preference for yellow, it is found equally in BC on Dalmatian and yellow toadflaxes.
Collection for redistribution
Plants can be swept for adults from June through September. Aspirating is most effective and less damaging to the plants.
R. neta was formerly known as Gymnaetron netum and Gymnetron netum and is currently also known as R. netum.
When existing on the same plants in their natural European habitat with R. antirrhini, R. neta is 50% larger.
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
Gassmann, A. and C. Paetel. 1998. Gymnetron netum (Col.; Curculionidae) a potential agent for biological control of toadflax in North America. CABI Bioscience, Div. CAB Internat., Switzerland. Summary Report 1996-1998. 43 p.
MFR staff observations and comments
Smith, J. M. 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.