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How to identify it
- Adults are metallic bronze, flattened, tapered toward the rear and up to 5 mm long. Females are all one color; a male's head is slightly greener than the rest of its body. Look for them at the base of flowering St. Johnswort through the summer, most actively in the heat of the day.[1]
- Larvae are white with brown mouthparts, have inflated segments and reach 11 mm. They and their damage are hidden inside the roots, so a root has to be split to find them.[1]
- This is the only St. Johnswort biocontrol agent that feeds in the roots. Stems rising from attacked roots are stunted with fewer flowers, but other things cause that too, so finding an adult or a larva is the only sure confirmation.[1]
Look-alikes
| Look-alike | How to tell them apart |
|---|---|
| Klamathweed beetles (Chrysolina quadrigemina and C. hyperici) | The leaf beetles released on the same weed; their adults and larvae feed on the leaves and defoliate the plant rather than boring the roots.[1][3] |
| St. Johnswort inchworm (Aplocera plagiata) | A moth whose caterpillars defoliate the plant; the gall midge Zeuxidiplosis giardi, another agent, forms galls in the leaves instead.[1][3] |
Where it lives
Native to Europe, mostly in its drier, more southern parts. A French population was released in California beginning in 1950 and later in other western states, and the beetle is now established in five western states and in British Columbia, where the stock that finally took, in 1987, came from Idaho after transfers from California in 1955, 1964 and 1977 apparently failed. In North America it is found mostly in mountainous regions. It is dispersed widely but populations are typically low with occasional explosions; in California it is widespread on the weed.[1][2]
Biology and life cycle
Larvae overwinter inside St. Johnswort roots and keep feeding the following spring as the plants bolt, then pupate in the root after the fourth larval stage. Adults emerge through the summer while the weed is in flower, and eggs are laid near the base of the plants in late summer. The new larvae burrow into the roots to feed and overwinter. One generation a year; UC IPM puts the larval feeding period from August to the following May or June and adult activity in July to early August.[1][2]
Larvae are susceptible to fungal attack, so populations often do poorly at damp sites. The beetle prefers large, multi-stemmed plants and dry mountainous sites, and it will attack plants in shade, which some of the other St. Johnswort agents will not.[1][2]
Damage and signs
- Larval feeding can consume the root tissue completely. Stems from attacked roots and root crowns are stunted and carry fewer flowers, and in some cases the plant dies outright.[1][2]
- Its record is mixed and hard to read. In California it initially showed it could destroy St. Johnswort, but the Klamathweed beetle overshadowed it; it still persists in some California locations with substantial impact, and UC IPM credits the two beetles together with keeping the weed at low densities there. Plant infestation levels reached 87 percent at one point in Washington, and the beetle contributed significantly to suppression in Idaho, where it is still abundant. Populations now vary across the Pacific Northwest and it is typically less common in British Columbia. Because there were no baseline data and Chrysolina beetles were abundant at its release sites, its overall impact is unknown.[1][2]
- The root borer has been observed attacking Hypericum concinnum, a forb or subshrub endemic to California and related to the weed; long-term effects on that plant have not been recorded.[1][2]
Health risk
A root-boring beetle of common St. Johnswort; none of the sources describes any risk to people.[1][2][3]
Control
Prevention
Pick a site the beetle can live on: it does poorly at damp sites where fungus attacks the larvae, and the beetles in general perform poorly at shaded, barren or rocky ground, although the root borer tolerates shade better than the other agents and prefers dry mountainous sites with large, many-stemmed plants.[1][2]
Biological control
The borer is a weed-control agent and needs no control. In California, UC IPM says the root beetle and the Klamathweed beetle are probably already present at most sites, that it can take several years for beetles to find a new infestation, and that adults can be collected for redistribution by sweep net on hot days from July to early August; the county advisor or CDFA's Biological Control program can help.[2]
Common questions
- My St. Johnswort is stunted and barely flowering. Is the root borer in it?
- Maybe, but stunting has other causes. Dig and split a root crown: white, segmented grubs up to 11 mm, or hollowed root tissue, confirm the borer, and in summer the small bronze adults sit at the base of the flowering plants.[1]
- Which is doing more good on my St. Johnswort, this beetle or the leaf beetles?
- UC IPM lists both the Klamathweed beetle and the root borer as the agents with the most impact and credits the pair with holding the weed at low densities in California; NAISMA notes the root borer's own contribution has been hard to separate because the leaf beetles were abundant at the same sites.[1][2]
- Does it attack anything but St. Johnswort?
- It has been observed attacking Hypericum concinnum, a related native plant found only in California; long-term impacts on that species have not been recorded.[1][2]
Sources
- North American Invasive Species Management Association (PDF hosted on Bugwood), “Randall (USDA Forest Service), Andreas (Washington State University Extension), Milan (Bureau of Land Management) and Moffat (Agriculture and Agri-Food Canada). Common St. Johnswort Biocontrol Agents: History and Ecology in North America. In: Winston, ed., Biological Control of Weeds in North America, NAISMA-BCW-2024-18-COMMON ST JOHNSWORT-A (2024).” bugwoodcloud.org/resource/files/33586.pdf (accessed 2026-11-10)
- University of California Statewide IPM Program, “Weed Control User Tool (WeedCUT): Biological Control of St. Johnswort.” weedcut.ipm.ucanr.edu/biological-control/st-johnswort/ (accessed 2026-11-10)
- Washington State Noxious Weed Control Board, “Common St. Johnswort (Hypericum perforatum).” www.nwcb.wa.gov/weeds/common-st-johnswort (accessed 2026-11-10)
- iNaturalist, “Agrilus hyperici taxon page (order and family).” www.inaturalist.org/taxa/987161 (accessed 2026-11-10)