Chrysolina hyperici and Chrysolina quadrigemina are very similar in appearance. Eggs are long and orange and are laid singly or in clusters on plant leaves. Larvae are approximately 6 mm in length hatching orange and turning to gray as they mature. Of similar length, adults are oval, plump and metallic shining green, blue or bronze in the sunlight.
Feeding on St. Johnswort makes larvae photosensitive so they often are only active before sunrise. Adults also retreat to the soil late in the summer. Prior to this, they congregate at the tops of plants, which will appear defoliated and wilted. They prefer sunny locations to shaded infestations.
Klamathweed beetles can overwinter as eggs, larvae or adults depending on climate. Typically, larvae from overwintered eggs emerge in the early spring and begin feeding on St. Johnswort foliage. Larvae develop through four instars before pupating in the soil in late spring. Adults emerge and continue feeding on foliage about the time the plant starts to flower. Adults return to the soil until they are ready to lay eggs. Egg laying is dependent on moisture. If there is enough rain in the fall, adults will lay eggs on plant tissue prior to the onset of winter. If there is not enough moisture, they will wait until spring to lay eggs, overwintering in the soil. In drier climates like Montana, adults usually wait until spring to lay eggs. There is one generation per year.
Severe defoliation by adults and especially larvae, reduce plants ability to photosynthesize, store carbohydrates and produce seeds.
Klamathweed beetles are readily available in Montana. If you are interested in obtaining these insects view the biocontrol vendor list for options.
C. hyperici is more tolerant of cold and wet climates than C. quadrigemina. Both are effective at reducing St. Johnswort and are often found in the same location, however, it is believed that C. quadrigemina is more effective than C. hyperici In Western North America.
Using the Agent
Due to their abundance, often introduction of Chrysolina spp. is unnecessary. If they are not present already, adults can be collected from flowering plants using a sweep net. At least 200 adults should be released to establish the beetle. Redistribution can be monitored by observing larvae, which feed at night. Adults may also be observed on sunny days in early summer. Using these beetles for biocontrol is most effective, when infestations are large, dense and other management options aren’t available. When possibly other IPM strategies should be employed.
St. Johnswort is a deep-rooted weed that can spread by root fragments so hand-pulling is most effective early in establishment but can encourage production later on. For more mature infestations, mowing can be effective to reduce seed production and weaken root system but this will work against the biocontrols as they are mostly found on above ground material at the ideal time to mow plants. Chemical controls can be used around the perimeter of infestations; however, the plant’s foliage can be waxy so a surfactant may be necessary. For more information on control methods and herbicide recommendations contact your local weed district or Extension Service.
Winston, R., C.B. Randall, R. D. Clerck-Floate, A. McClay, J. Andreas, M. Schwarzlander. May 2014. Field Guide for the Biological Control of Weeds in the Northwest. http://www.ibiocontrol.org/westernweeds.pdf
Washington State University Integrated Weed Control Project. Biological Control Agents. Accessed January 18, 2017. http://invasives.wsu.edu/biological/chrysolinaspp.htm