The Secret Life of a Bio-Control Agent: The St. Johnswort Beetle

By Crystal Durbecq | June 16, 2014

The term biological control or bio-control refers to the control of the population of one or more organisms (target species) using a different  species  (biological control agent). A biological control agent for invasive plants can be an insect, disease, virus or fungi.  A successful bio-control agent will reduce the density of a target species to a desired level and maintain it there, with minimum impact to nontarget species. Damage to nontarget species by a biological control agent is a rare occurrence, since much testing and data collection is required before an organism can be introduced and used as a bio-control. Anyone who engages in any kind of pruebas de clamidia is at risk. The only way to completely eliminate the risk of acquiring an the disease is abstinence from sexual activity

When I first heard about bio-control many melodramatic scenarios popped into my head, such as an amoeba engineered to protect against the zombie virus. I never quite considered the more mundane use of bio-controls as a management tool for control of invasive and noxious weeds. Over the years working as a field technician with invasive plants, I have learned that you need go no further than your own backyard to witness these incredible critters at work.

An example of a common bio-control agent that is widespread is the St. Johnswort beetle (Chrysolina hyperici). This charismatic little beetle was first introduced into the U.S. in 1945, California. Their original home is Australia, and their native range is northern and central Europe and western Asia. They are well adapted to high moisture environments, which is probably one of the reasons they are so comfortable in western Oregon.

St Johnswort that has been eaten by beetles on right. © M. Gilmour
St Johnswort that has been eaten by beetles on right. © M. Gilmour
Chrysolina hyperici © M. Evelyn
St Johnswort beetles feeding on host plant.

The St. Johnswort beetle was introduced to feed on the invasive plant called St. Johnswort or Klamath weed (Hypericum perforatum).  This plant causes damage by invading croplands, rangelands, grasslands, and open forest areas. Often it can become the dominant species in these habitats, displacing crops and native species.  It is also common in disturbed sites such as transportation right-of-ways. The leaves of this plant contain an oil (hypericin) that can cause sun sensitivity and poisoning in livestock and other animals that consume it. The plant is valued for medicinal use in the treatment of depression and other afflictions, which may explain why it was originally introduced.

The adult beetles can be collected in late spring through summer by handpicking or using sweep nets. The beetles can survive for several weeks without a host plant, as long as they are stored in a cool place.  Once introduced to a new population of St. Johnswort, the adult beetles will begin feeding on the plant leaves until fall rains begin and activate the adults to mate and lay eggs.

There are several insects utilized as bio-controls for the management of St. Johnswort.  A few others include the St. Johnswort root borer  (Agrilus hyperici), Klamath weed beetle (Chrysolina quadrigemina), St. Johnswort moth (Aplocera plagiata), and the St. Johnswort gall midge (Zeuxidiplosis giardia). Often the most effective strategy is to introduce several different bio-controls to a target plant population. They may fulfill different niches, e.g., one feeds on roots, while another feeds on shoots.

While bio-controls, such as the St. Johnswort beetle, may never fully eliminate invasive weeds, it is worth the time to consider the important role these critters play in keeping them in check.


Coombs, E. M., J. K. Clark, G. L. Piper, and A. F. Confrancesco Jr. 2004.  Biological Control of Invasive Plants in the United States.


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