Chrysolina quadrigemina are leaf beetles (Chrysomelidae) that are native to Europe and North Africa. They were imported in the mid forties as the first attempt in the US at biocontrol of invasive weeds with insects, in this case Hypericum perforatum (known in our region as Klamath Weed or St. John’s Wort), which was taking over rangeland in the West, and is toxic to livestock.
They are an effective biocontrol, having evolved with this Hypericum sp. and timing their life cycles to its cycles. Adults emerge from pupation in early spring, as the plant is leafing out. They feed heavily upon new growth, including buds and flowers, until the hot and dry part of summer, when they go into aestivation (think of it as summertime hibernation), having effectively denuded the plant. With the advent of fall rains they emerge to feed upon, and lay eggs upon, the basal foliage that new plants are forming. The larvae continue feeding on the basal leaves until cold weather, when they enter the soil and pupate until the following spring.
Unfortunately summer rains can disrupt this cycle, making Klamath Weed Beetles less effective as a biocontrol agent. For this reason C. hyperici, which is more tolerant of summer moisture, were also introduced in the wetter parts of our region. These two beetles are basically impossible to differentiate in the field. C. quadrigemina tends to be bigger (up to 7.1mm) but there is much overlap, and darker (again, much overlap). A small, light C. quadrigemina and a large, dark C. hyperici are indistinguishable without magnification.
But under a microscope one can see that on Chrysolina quadrigemina the fine elytral puntures (which surround the larger elytral puntures) are equal to or finer than the punctures on the pronotum, whereas in C. hyperici they are coarser than those on the pronotum. If the specimen is a male then they can also be differentiated by the fact that there is a depression on the fifth visible sternite of C. quadrigemina that is lacking on C. hyperici.
Habitat- Wherever there is Hypericum perforatum
Range- Region Wide
Eats- Adults and larvae feed only on Hypericum sp.
Flight Season- Adults active April through early July, and again between first rains of fall and the first hard frosts.