Cornell University, Biological Control: A Guide to Natural Enemies in North America

Gymnetron linariae adult.
C. Paetel, CABI Biosciences

Gymnetron linariae Panzer
(Coleoptera: Curculionidae)

by Rich Hansen, USDA-APHIS-PPQ, Forestry Sciences Lab, Montana State University, Bozeman, MT 59717-0278.


Dalmatian toadflax infestation:
W.Hartung, NRCS;
and plant (inset):
R.Hansen, USDA-APHIS

Gymnetron linariae was approved for release in the US in 1995. Several releases have been made in western Canada and the US; small populations may be established in Canada, but no confirmed establishment has yet been reported in the US. (Background information about Dalmation toadflax and yellow toadflax is available.)

Appearance

    Gymnetron linariae adults are small, black, oval weevils about 2 mm long, with a noticeable snout. They may be found feeding on toadflax shoots.

    Larvae are small, legless, and C-shaped when viewed laterally; presumably, they are creamy-white in color with a yellowish head capsule as are larvae of other Gymnetron spp. Larvae are found within galls on toadflax roots.

Habitat

    Grasslands, pastures, agricultural fields, and roadsides infested with yellow or Dalmatian toadflax.

Pests attacked

    Gymnetron linariae attacks both Dalmatian and yellow toadflax. Based on host specificity tests, only a few Eurasian Linaria spp. are suitable hosts for this weevil.

Life cycle


    G. linariae root galls on toadflax plant.
    C. Paetel, CABI Biosciences

    Gymnetron linariae adults appear in late spring, and apparently live for up to several months. After several weeks of feeding on young toadflax shoots, adults mate and females begin laying eggs in toadflax roots. Eggs are deposited singly into small pockets chewed into the root tissue by the female weevil and covered with excrement. Most eggs are laid in the root crown area, within several inches of the soil surface.

    In response to female oviposition and, perhaps, larval feeding, round or oval galls are formed on toadflax roots within which G. linariae larvae feed and develop. Up to 200 galls may be found on a single toadflax plant; individual galls may fuse to form "gall masses" consisting of 40 or more galls. Larval development consists of three stages and takes two or three months.

    Pupation occurs within galls in late summer. Some newly-eclosed adults may remain within galls and enter diapause, while others may emerge and feed on toadflax shoots for a week or two before returning to the soil to overwinter.

    There is one generation per year.

Relative effectiveness

    The potential impact of Gymnetron linariae on toadflax plants is not yet known. Presumably, root galls will reduce plant vigor by acting as a nutrient "sink," as has been suggested for other plant-galling insects.

Pesticide susceptibility

    Unknown.

Conservation

Commercial availability

    Gymnetron linariae is not yet generally available.

Reference

    Jordan, K. 1994. Gymnetron linariae. Panzer (Col., Curculionidae), a candidate for biological control of Dalmatian and yellow toadflax in North America. Intl. Inst. of Biol. Control European Station Final Report. 37 pp.

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