Rhinusa antirrhini (formerly Gymnetron)
<<back to yellow toadflax
Common Name
Toadflax seed capsule weevil
Weed Impacted
Larvae inhibit reproduction of yellow toadflax by feeding on immature seeds. Seed production was reduced by 90% at some sites in Washington, but elsewhere (Canada) reductions of 20 to 25% have been more typical. Adults may affect the growth of toadflax plants by feeding on flowers and young shoots.
Insect Description
Adults are small, black, oval-shaped weevils approximately 2.5 mm (1/16 to 1/8 in) long.
Life Cycle
Adults are typically active from May or June until August. Females lay eggs in the ovaries of toadflax plants while the plants are flowering. Immature seeds surrounding the eggs become swollen and watery. Young larvae begin feeding on swollen seeds; mature larvae may also consume normal seeds. Pupation occurs within the seed capsules. New adults emerge in August and September, feed for a short period of time, and then overwinter either in soil litter beneath the host plant or within the seed capsules where they developed.
Field Identification
Several beetle species have been approved for biological control of toadflax, and sometimes field identification can be difficult. If different agents occur together, weevils can be distinguished from non-weevils (i.e. Brachypterolus pulicarius) by the presence of a snout. Rhinusa weevils are much shorter and rounder than Mecinus janthinus, a bluish-black weevil more common on Dalmatian toadflax. Rhinusa antirrhini can then be distinguished from another Rhinusa species, R. neta, by its darker color. R. antirrhini is black, or nearly so, whereas R. neta is lighter gray or drab-colored.
Availability
This beetle is widespread and usually common wherever yellow toadflax occurs, so redistribution may be unnecessary.
Comments
Releases of what was thought to be a Dalmatian toadflax-adapted strain of this weevil (now considered a separate Rhinusa species) were made in Wyoming in 1998. However, to date, it is unknown whether that population or any others have established on Dalmatian toadflax.
Using the Agent: Integrated Weed Management (IWM)
Because R. antirrhini is already found at most yellow toadflax sites, redistribution may be unnecessary. Using R. antirrhini with the flower-feeding beetle Brachypterolus pulicarius may increase the impact on seed production.
Agents should only be released at sites where toadflax infestations are large and immediate eradication of the weed is not the primary objective. Smaller populations of toadflax can be either hand-pulled or sprayed, but treatments must be repeated annually for up to 5 or 6 years. Viable root fragments can remain after pulling, and herbicides will not immediately kill or penetrate all plants. If spraying larger infestations, herbicides should be applied only after insect agents have definitively established. Consult with your county weed office, Extension office or an IWM practitioner to determine which herbicide treatments are safe and appropriate.
Burning is not recommended, as it increases the susceptibility of a site to further infestation. Plowing or disking can eliminate toadflax in cropland if repeated regularly. Mowing will not control toadflax and can work against any potential agent impacts when the insects are active. Plants should be mowed only if necessary either before May or in the fall when insects are not feeding or developing on the plants.
Any management of toadflax should involve revegetation with native or other desirable plants. Revegetation promotes competition between plants, which stresses weeds and allows for more effective treatment. To learn more about revegetation and IWM, click here.
References
Coombs, E. M., J. K. Clark, G. L. Piper, and A. F. Cofrancesco, Jr. 2004. Biological Control of Noxious Weeds in the United States. Western Society of Weed Science, Oregon State Univ. Press, Corvallis.
Wilson, L. M., S. E. Sing, G. L. Piper, R. W. Hansen, R. De Clerk-Floate, D. K. MacKinnon, and C. Randall. 2005. Biology and Biological Control of Dalmatian and Yellow Toadflax. USDA Forest Service. FTET-05-13.
Photo Credit
Rhinusa antirrhini adult - Bob Richard, USDA-APHIS-PPQ, bugwood.org (insectimages.org), http://www.insectimages.org/images/768x512/1416043.jpg
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