Six species of insects that feed on St. Johnswort were approved for introduction, five of which established. The leaf beetle, Chrysolina quadrigemina (Coleoptera: Chrysomelidae), and the root borer, Agrilus hyperici(Coleoptera: Buprestidae) have the most impact. The leaf beetle, Chrysolina hyperici (Coleoptera: Chrysomelidae), the inchworm, Aplocera plagiata (Lepidoptera: Geometridae), and the gall midge, Zeuxidiplosis giardia (Diptera: Cecidomyiidae), have little or no impact.

Two similar species of leaf beetles, Chrysolina hyperici and C. quadrigemina, are established in California, and the latter species (also known as the Klamathweed beetle) is the most abundant of these two species in California (Figs. 1 & 2). They have similar life cycles, but C. quadrigemina does better at drier sites and emerges earlier in spring than C. hyperici. In California, C. hyperici has become relatively uncommon and is found only in extreme northwest California near the border with Oregon. Both species lay eggs in the fall on the undersides of leaves. Larvae feed on the leaves and can completely defoliate plants. Pupation occurs in the ground (Feb. - Mar.). Adults emerge in the spring, feed on leaves and flowers for several weeks, then they hide in the soil during summer until fall rains begin.

Larvae of the root borer attack the roots from August to the following May or June (Fig. 3). Larval damage stunts the stems and reduces flower production, and many attacked plants die. Adult beetles are active from July to early August and can be collected by sweep net (Fig. 4. The root borer is widespread on St. Johnswort in California, but larvae are susceptible to fungal attack at damp sites.

The inchworm is a defoliating moth that can have up to two generations per year (June to July and Sept. to May) (Fig. 5). Defoliation weakens plants and reduces seed production. Larvae are primarily active at night and hide in the soil to pass the winter. In California, it occurs in just one location near Mt. Shasta in Siskiyou County.

The gall midge forms galls in leaf buds and has 2-3 generations per year (Figs. 6 & 7). It is rarely recovered in California, and parasites limit its ability to build up populations.

Biological Control Agents


Common name




Agrilus hyperici

St. Johnswort root borer



First release in CA in 1950

Aplocera plagiata




First release in CA in 2011

Chrysolina hyperici

leaf beetle



First release in CA in 1945

Chrysolina quadrigemina

Klamathweed beetle, leaf beetle



First release in CA in 1946

Zeuxidiplosis giardi

gall midge



First release in CA in 1950

How the Technique Is Employed

The Kamathweed beetle quickly eliminated vast infestations of St. Johnswort in California during the late 1940s. Together with the root beetle, these two species continue to maintain the weed at low densities. It may take several years for beetles to discover new weed infestations.

Biological control agents may be present at your site. Look for signs of leaf damage by beetle larvae during late winter or by adults in the spring. If you fail to see signs of the beetles, then collect them from other sites to release. Adult Klamathweed beetles can be collected by sweep net or hand picking in May when the plant is flowering. During summer, the beetles rest in the ground and are harder to collect. Keep them cool on fresh stems with leaves and release them as soon as possible.

Collect adult root borer beetles by sweep net on hot days from July to early August.

Larval inchworms become active at night and can be collected by sweep net in midsummer or fall at, or immediately after, sunset.

Collect inchworm larvae (April, June or September) by sweep net.

The gall midge is difficult to transfer. Infest potted plants and transplant them at the release site to allow adults to emerge from galls.

For more details, see Winston et al. (2012) listed below.

Special Tips

Focus on the agents that are known to perform well in your area (discuss with county advisors and other land managers).

The beetles perform poorly at shaded, barren or rocky sites, whereas the defoliating moth (inchworm) does well at such sites.

While the gall midge does best at more humid sites and at higher elevations, it is rarely recovered in CA.

The root borer prefers dry mountainous sites and tolerates shade better than the other species.


The two Chrysolina beetles have been reported to attack the native plant Hypericum concinnum and the introduced ornamental H. calycinum but do not appear to affect their populations.

The root borer has been reported to attack a native plant, Hypericum concinnum.

The gall midge is able to form galls on the native plant Hypericum concinnum, but damage to this plant species is insignificant.

Where Can I Get These?

The two most effective agents, the Klamathweed beetle and the root beetle, are probably already at your site.

Contact your county advisor or CDFA Biological Control program.


Piper, G.L. 2004. St. Johnswort. In: E.M. Coombs, J.K. Clark, G.L. Piper, and A.F. Cofrancesco, Jr. (eds.), Biological Control of Invasive Plants in the United States. Oregon State University Press, pp. 322-334.

Winston, R., C.B. Randal, M. Schwarzländer, and R. Reardon. 2012. Biology and Biological Control of Common St. Johnswort. USDA Forest Service, Forest Health Technology Enterprise Team. FHTET-2010-05, 2nd edn., May 2012. i-v, 96 p. https://www.fs.fed.us/foresthealth

Contributing Authors

Dr. Michael J. Pitcairn, Program Manager, California Department of Food and Agriculture

Dr. Lincoln Smith, Research Entomologist, USDA