Sphenoptera jugoslavica
<<back to diffuse knapweed
Common Name
Bronze knapweed root borer
Weeds Impacted
This beetle prefers diffuse knapweed, but will also attack spotted and squarrose knapweed. Larvae bore into and tunnel within knapweed roots, reducing the availability of the plant’s energy reserves.
Insect Description
Adults are dark, metallic bronze beetles, 8-10 mm (approximately 3/8 inch) long, and somewhat flattened. Larvae are whitish grubs found within the roots. They have a distinctly widened head that distinguishes them from all other knapweed root boring insects.
Life Cycle
Adults are active from mid-July through August and feed for two to three days before mating. Females lay eggs during July and August in the base buds (axils) of rosette leaves. Young larvae feed in the axils and then migrate through the plants into the roots. Generally, one larva develops in a single root. Larvae overwinter and pupate within the root, emerging as adults the following summer.
Field Identification
Several insects have been released for control of knapweeds, but S. jugoslavica is easily distinguished by its elongate, flattened form and metallic color. Larval feeding causes gall-like enlargements in the roots, and feeding tunnels will often be filled with sawdust-like frass. If S. jugoslavica occurs with other root-feeding insects (Agapeta zoegana and Cyphocleonus achates), S. jugoslavica larvae can be easily distinguished by a noticeable widening toward the head. They will also generally be smaller than C. achates larvae.
Availability
This beetle is readily available in Washington and Oregon. Collections are made beginning around mid-July. To inquire about availability and to request this biocontrol agent, please download, complete and submit this form.
Comments
S. jugoslavica has been shown to significantly decrease knapweed biomass, seed output, and plant density.
Using the Agent: Integrated Weed Management (IWM)
Releases of S. jugoslavica should only be made at sites where knapweed infestations are large and immediate eradication of the weed is not the primary goal. Knapweed seed head insects and/or other root feeders may already be present at a given site and S. jugoslavica can be used in combination with all available agents.
Smaller, satellite knapweed populations 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. Herbicide applications on larger infestations should only be made after insect agents have definitively established, and should be timed according to the life cycles of both the plant and the insect.
Mowing will not control knapweed. While it may not affect S. jugoslavica or other root-feeders, mowing when plants are flowering interferes with the success of seed head insects. Repeated grazing in the spring by sheep and goats (not horses) can weaken plants; but like mowing, grazing while plants are flowering can be detrimental to seed head insect populations.
Prescribed burning can destroy above-ground parts, promote germination of existing seeds, and facilitate earlier treatment of developing plants. However, it also increases the susceptibility of sites to new invasions by knapweed or other weeds.
Any management of knapweed should involve revegetation with native or other desirable plants, especially grasses. 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 Invasive Plants in the United States. Western Society of Weed Science, Oregon State Univ. Press, Corvallis.
Seastedt, T. R., D. G. Knochel, M. Garmoe, and S. A. Shosky. 2007. Interactions and effects of multiple biological control insects on diffuse and spotted knapweed in the Front Range of Colorado. Biological Control 42(3):345-354.
Waldo, A. J. 2003. Identification and Control of Knapweed Species in Central and Eastern Oregon. Oregon State Univ. Extension Publication EC 1559. URL http://extension.oregonstate.edu/catalog/html/ec/ec1559/
Photo Credits
IWCP staff
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