In 1946, imported beetles were released against toxic St. Johnswort in northern California; by 1957, the weed had fallen to less than 1% of its former abundance

Common St. Johnswort, which is called Klamathweed by California ranchers, is a European and West Asian native that had become one of the worst rangeland weeds in California by the 1940s. Common St. Johnswort contains hypericin, which causes the animals to become sun‑sensitive, resulting in blisters, weight loss and even death in severe instances. UC IPM notes that it is a rangeland weed that was poisonous for grazing animals and had spread to such an extent that it was threatening vast areas of grazing lands in the northern region of California

Chrysolina hyperici and C. quadrigemina were chosen for their narrow diet on Hypericum

As Klamathweed lacked natural enemies in California, entomologists could use classical biological control: introducing an insect that feeds on related plants in its native range into a place where the plant has become a weed. Two very similar species of leaf beetles, Chrysolina hyperici and Chrysolina quadrigemina, were selected because both have a highly specialized feeding habit and mainly attack members of the Hypericum family. The entomologists responsible for establishing these insects on California rangeland were Carl B. Huffaker and C. E. Kennett of the University of California at Berkeley’s Division of Biological Control and the first introduction of C. quadrigemina took place in February 1946.


Klamathweed abundance fell by more than 99 percent by 1957

The best evidence of what happened in subsequent years is contained in the publication 'A Ten-Year Study of Vegetational Changes Associated with Biological Control of Klamath Weed' authored by Huffaker and Kennett in the Journal of Range Management. In the study, the authors followed the rangeland plots that suffered from severe Klamathweed infestation before the introduction of the beetles, noting changes in the weed's coverage during the course of ten years and its influence on the value of the land as a grazing territory. The study aimed to answer two questions: how much the beetles reduced the weed population and what that reduction was worth to landowners.

According to UC IPM, the weed population had shrunk to less than 1 percent of its original level within 10 years of the beetles' release, and the weed disappeared from many previously infested areas. The Huffaker and Kennett figures confirm this assessment. The decline continued after the beetles were introduced, and the suppression has held ever since, saving ranchers yearly losses on infested land.

C. quadrigemina's tolerance for drier sites made it the dominant species statewide

Chrysolina quadrigemina and C. hyperici have slightly different life cycles and those differences helped C. quadrigemina become the dominant species in some areas. Both species lay their eggs in the fall under Klamathweed leaves when the rains begin and the larvae feed through winter; they can sometimes completely defoliate a plant before pupating in the soil from February to March. Adult beetles emerge in spring, feed for several weeks, then bury themselves in the ground and remain there through the dry season, emerging again in fall when the rains return to lay eggs. UC IPM also notes that C. quadrigemina can live in drier areas and emerges earlier in the season than C. hyperici, which helps explain its wider distribution in California’s Mediterranean climate.

A 2016 study found spillover onto the native Hypericum punctatum

Several decades after its success in California, C. quadrigemina had an unintended effect in the eastern United States, where it had also been used against an invasive weed. In 2016, there was a publication called 'Spillover of a Biological Control Agent (Chrysolina quadrigemina) onto Native St. Johnswort (Hypericum punctatum)' by Jessica L. Tingle, Susan C. Cook-Patton and Anurag A. Agrawal in the journal PeerJ. This study analyzed how Chrysolina quadrigemina reacts to a native species of the genus. As the authors noted, while the beetle had successfully regulated the population of the exotic Hypericum perforatum in California, in the eastern United States, it actively feeds on another native species of the same genus called Hypericum punctatum, which was discovered based on direct observations of the beetles feeding on the native species.

However, in contrast to the exotic species, the spillover of C. quadrigemina on H. punctatum did not result in limiting the range of this native plant to the forest edge, as it is still observed in bright areas. Another feature of this study includes the possibility of the beetle serving as a vector of the fungal pathogen Colletotrichum gloeosporioides, which is a potential factor in regulating the exotic species of Hypericum. Taken together with the earlier findings, this later study is a reminder that even a biological control program celebrated as one of the most successful of its kind is not free of unintended consequences once an agent is established across a wider range.

The beetles remain established on California rangeland and Ornamental Hypericum today

After more than seven decades since their introduction, Chrysolina hyperici and Chrysolina quadrigemina continue to thrive in all areas where Klamathweed thrives in California and the level of suppression accomplished has been maintained. The story doesn't stop there, however. There are many ornamental Hypericum species, including creeping St. Johnswort, that are now common as ground cover in California gardens and these are attacked by the same beetles. The plants in irrigated gardens can tolerate the herbivory with only cosmetic effects on the plant.

From the example of the Klamathweed collapse, the wider lesson learned that still informs the biological control program today is how effective the introduction of one well-suited insect to an invasive plant species could be in making the plant insignificant in less than ten years. Also, the subsequent discovery of the effect of the introduced insects on the native relative of the plant elsewhere is evidence of the importance of the specificity test and monitoring before the introduction of the species to control the other one.