Exapion fuscirostre 
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Common Name
Scotch broom seed weevil
Weed Impacted
Weevil larvae may suppress the spread of Scotch broom by feeding on seeds within developing seedpods. In Oregon, 40 to 60% of seedpods are attacked and seed reduction in attacked pods averages 85%. Adults feed on flowers and on the tips of twigs but their impact is insignificant.
Insect Description/Field Identification
Adults are small, dark gray weevils, 2 to 3 mm (approximately 1/16 to 3/32 inch) long, with brown legs and light gray or silver bands extending down both sides of the back. Larvae are white with a brown head-capsule, C-shaped and found feeding externally on the developing seeds within the seedpods. Comparatively, the larvae of B. villosus are not readily visible as they feed within the developing seed, creating a translucent-like appearance of the seed.
Life Cycle
Adults can be found throughout the year and overwintering adults may be present when the first-generation adults emerge from pods in the spring. Females feed on flowers, then lay eggs into seedpods. Eggs hatch in one to two weeks. Larvae feed on the exterior region of the developing seeds, usually with a single larva feeding on a single seed. Larval development takes 20 to 40 days. Pupation occurs within the seedpods and lasts from 10 to 20 days.
Availability
This beetle is available in Washington, Oregon, and California, although its overall effectiveness in reducing Scotch broom stand densities is uncertain. Adults are collected in April and May after they have mated. To inquire about availability and to request this biocontrol agent, please download, complete and submit this form.
Comments
Exapion fuscirostre prefers open meadows and south-facing hillsides. The adults can establish and disperse rapidly, up to 2 km (about 1 1/4 mile) per year from the site of release. Parasitoids regularly attack both E. fuscirostre and B. villosus, although their impact on the biocontrol agent populations are currently unknown. Indications of parasitism are noticeable by small emergence holes on the pod exterior.
Using the Agent: Integrated Weed Management (IWM)
This weevil can be used in combination with the Scotch broom beetles (Bruchidius villosus). Current studies suggest that B. villosus may be a more effective agent. Biocontrol agents should only be released where infestations of Scotch broom are large and immediate eradication is not the primary objective. Most treatments are not directly compatible with the use of these biocontrol agents. Any control methods that prevent flowering and seed production will reduce or eliminate biocontrol populations. Biocontrol agents can be used within large patches and satellite patches (which lead to increased spread) can be controlled using other management tools.
Smaller infestations and younger plants can either be dug or hand-pulled using tools such as a Weed Wrench ™. Cutting older stems (greater than 2” in diameter) at the base can effectively eliminate some Scotch broom plants, particularly if they are stump treated.
Control of Scotch broom can be difficult and requires an integrated strategy. Herbicides can be effective, if used on regrowth from mowed or cut Scotch broom, in fall and the following spring. Consult with your county weed office, Extension office or an IWM practitioner to determine which herbicide treatments are safe and appropriate.
Any management of Scotch broom should involve revegetation with native or other desirable plants. Revegetation promotes competition between plants, which can stress weeds and allow for more effective treatment. Method of control will determine when, where, and how often sites will need to be reseeded or replanted. 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.
PNW Extension Publications. 1998. Scotch Broom. PNW Extension Bulletin 104.
Photo Credit
Exapion fuscirostre - IWCP staff
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