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Gymnetron netum (Germar) by Rich Hansen, USDA-APHIS-PPQ, Forestry Sciences Lab, Montana State University, Bozeman, MT 59717-0278. Gymnetron netum was accidentally introduced into North America in the early 1900's, presumably as a contaminant of imported ornamental toadflax plants. G. netum has been reported from a number of states across the northern US and from British Columbia. Most collections were reported from yellow toadflax, though this species has also been collected from Dalmatian toadflax in the northwestern US. A presumed Dalmatian toadflax-specific "strain" of G. netum is under investigation but has not yet been approved for release in the US. (Background information about Dalmation toadflax and yellow toadflax is available.)
Appearance Gymnetron netum adults are oval, dark grey beetles about 2-3 mm long that have a noticeable snout. They may be found on young toadflax shoots. G. netum larvae are 2-4 mm long, legless, and C-shaped when viewed from the side. They are creamy-white in color with a light brown head capsule. Larvae are found within toadflax seed capsules.
Habitat Grasslands, pastures, agricultural fields, and roadsides infested with yellow or Dalmatian toadflax.
Pests attacked Gymnetron netum attacks both yellow and Dalmatian toadflax; there may be "strains" of this insect that preferentially attack a single toadflax species. Studies examining the host specificity of this species are currently in progress.
The life cycle of G. netum appears to be quite similar to that of G. antirrhini. Adult weevils emerge in late spring or early summer, and feed on young toadflax shoots and buds for a week or two. After mating, female beetles lay eggs inside toadflax flowers, inserting them singly through the ovary wall. Females lay a total of about 20-60 eggs. Oviposition causes abnormal growths within the ovary, and ovules and young seeds adjacent to these growths become distended and yellowish. Larvae hatch from eggs and feed on these "inactivated," abnormal seeds. Larval development consists of three stages and last about three to five weeks. Mature larvae construct cells within developing seed capsules and then pupate. In several weeks, adults eclose and may feed on toadflax shoots for a short period of time before entering diapause. Winter is spent as an adult in plant litter or within old toadflax seed capsules. There is one generation per year.
Relative effectiveness Gymnetron netum feeds on developing seeds and will, presumably, reduce seed production on attacked plants, as documented for G. antirrhini. This agent will not directly kill toadflax plants, and long-term effects on toadflax abundance are not yet known. Gymnetron netum may be displaced by populations of G. antirrhini when the two species occur together.
Pesticide susceptibility Unknown.
Conservation Specifics are unknown. For general information about conservation of natural enemies, see Conservation in the Tutorial section on this site, Feature Article on conservation in Volume II, No. 1 of Midwest Biological Control News.
Commercial availability Gymnetron netum may possibly be collected from some yellow toadflax stands in the US.
References Paetel, C. 1997. Investigations on potential biocontrol agents of Dalmatian and yellow toadflax, Linaria dalmatica and L. vulgaris. Intl. Inst. of Biol. Control European Station Annual Report. 22 pp. Smith, J.M. 1959. Notes on insects, especially Gymnaetron spp. (Coleoptera: Curculionidae), associated with toadflax, Linaria vulgaris Mill. (Scrophulariaceae), in North America. Can. Entomol. 91: 116-121.
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