Chrysolina spp.
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Common Name
Klamath weed beetles
Weed Impacted
The larvae and adults of two species of Chrysolina (C. quadrigemina and C. hyperici) feed on the leaves of St. Johnswort (Klamath weed, goatweed). C. quadrigemina, is known to feed on one ornamental and one native St. Johnswort, but no broad-level impacts have yet been reported on the native species.
Insect Description
Adult beetles are shiny metallic green, bronze, blue, or black; oval; and from 5 to 7 mm (approx. 1/4 in) long. Larvae are fat and humpbacked in appearance. They are usually orange at first, turning grayish-pink as they mature.
Life Cycle
Adults usually emerge late May to early June, feed until about mid-July, and then enter the soil to rest for the summer. In California and Oregon, fall rains stimulate the beetles to re-emerge, mate, and lay eggs. In drier areas like the inland Pacific northwest, spring rains the following year will induce mating. Females lay eggs on the leaves singly or in clusters, and the eggs hatch after about a week. Young larvae feed on leaf buds and immature leaves and can completely defoliate a plant before they reach maturity. Mature larvae burrow into the soil where they create cells and pupate. Beetles can overwinter as eggs, larvae, or occasionally as adults.
Field Identification
The presence of Chrysolina spp. on St. Johnswort can be indicated by naked stems and/or perforated leaves. The metallic color of adults and humpbacked appearance of the larvae make these beetles almost unmistakable in the field.
Availability
Chrysolina spp. are generally widely available but this varies from year to year. To inquire about availability and to request this biocontrol agent, please download, complete and submit this form.
Comments
C. quadrigemina was introduced into the United States in 1946 as one of the first insect agents to be used for the biological control of weeds. Its early successes in California have made it one of the most well-known bioagents used in weed control. Despite their celebrity, however, Klamath beetles may not be able to suppress St. Johnswort infestations at every site. C. quadrigemina and C. hyperici do best in areas with strong fall rains, when eggs can be laid and larval feeding can weaken plants before winter. In drier areas where eggs are laid in the spring, plants can sometimes outgrow feeding injury, especially where rain is common in the summer.
Using the Agent: Integrated Weed Management (IWM)
Chrysolina spp. have established at many sites across several states, so redistribution may be unnecessary. At sites where the beetles are not present, other St. Johnswort agents may be better adapted and thus better suited for release (e.g. Aplocera plagiata in eastern Washington, eastern Oregon, and northern Washington).
Releases should only be made at sites where St. Johnswort infestations are large and immediate eradication of the weed is not the primary objective. Smaller weed populations and satellite plants can either be hand-pulled or sprayed. 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 can actually increase plant growth and density. Mowing is not an effective means of control and is generally not recommended. Mowing while insects are active will work against any potential agent impacts on the weed, either by destroying the agents outright or by removing their source for growth and development.
Any management of St. Johnswort should include revegetation with native or other desirable plants. Revegetation promotes competition between plants, which stresses weeds and allows for more effective treatment. For more information about revegetation, click here.
References
Coombs, E. M., J. K. Clark, G. L. Piper, and A. F. Cofrancesco, Jr. 2004. Biological Control of Invasive Plants in the United States. Western Society of Weed Science, Oregon State Univ. Press, Corvallis.
Photo Credit
Chrysolina spp. adult - Cheryl Moorehead, bugwood.org (insectimages.org), http://www.insectimages.org/images/768x512/5211089.jpg
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