Klamathweed beetle—Chrysolina quadrigemina
The Klamathweed beetle, a type of leaf beetle in the family Chrysomelidae, was deliberately introduced into California during the 1940s to control common St. Johnswort, Hypericum perforatum. Also called Klamathweed, this inadvertently introduced rangeland weed is toxic to cattle and sheep. The beetle largely eliminated Klamathweed from several hundred thousand acres in California. Each year this saves ranchers millions of dollars in otherwise lost grazing land and poisoned livestock.
Subsequently, other Hypericum species became popular ground covers and shrubs in California landscapes. These, especially creeping St. Johnswort, H. calycinum, are also fed upon by this leaf beetle.
Adults are oblong and about 1/4 inch long with a downward-pointed head. They are metallic shiny and colored blue, bronze, green, or purple. Eggs are oblong and about 1/25 inch long. Larvae are plump and somewhat humpbacked. They are gray, orangish, or pinkish and grow up to 1/4 inch long.
Klamathweed beetle develops through four life stages: egg, larva, pupa, and adult. Each adult female can lay about 1,000 eggs during her life span of several months. Eggs are laid from fall through winter singly or in clusters on leaves. The beetle overwinters primarily as eggs or larvae. After hatching, larvae feed during late fall through winter and develop through three, increasingly larger instars. Mature larvae (prepupae) enter soil to a depth of about 1 inch during late winter and form a cell in which to pupate.
Adults emerge primarily in April and May, mate, and mostly feed during spring and early summer. During hot, dry summer weather adults aestivate (are inactive and do not feed). Adults again become active in fall, feeding and laying eggs.
Adults and larvae chew and feed on various Hypericum species. Larvae feed on lower leaves and the base of stems during fall through winter. Adults feed higher on plants, and their chewing damage occurs primarily during spring and early summer when hosts produce most of their growth flush and flowering. After being inactive during summer, adults feed to a lesser extent again during the fall.
When Klamathweed beetle is abundant, ornamental species of Hypericum, especially creeping St. Johnswort, can be heavily defoliated. Plants in irrigated landscapes generally recover and the chewing damage is primarily aesthetic.
Provide regular, deep irrigation for hosts during the drought season, especially when temperatures are hot. Protect plants from injury, such as by avoiding compaction or other soil disturbances around roots. Vigorous host plants tolerate moderate leaf beetle feeding, and control is generally not warranted to protect plant health.
Removing litter accumulated beneath plants in hot areas may reduce the survival of adult beetles that rest there during the summer. Keeping soil beneath plants moist during the spring may increase disease and mortality of immature beetles that pupate then near the soil surface.
Applying parasitic (entomopathogenic) nematodes (e.g., Heterorhabditis or Steinernema spp.) to soil beneath plants may provide control if applications are made during late winter, shortly before larvae pupate. Monitor the lowest parts of host plants for larvae to determine when to apply nematodes; pupation occurs after mature larvae are observed on foliage near the ground.
If chewing damage has not been tolerable, foliar or systemic insecticides can provide control. Because foliar sprays should target young larvae, monitor basal stems and lower leaves for larvae regularly in fall and early winter if you are considering spraying insecticide.
Bacillus thuringiensis ssp. galleriae (e.g., BeetleGONE!) selectively kills leaf beetle adults and larvae that consume sprayed foliage. Because of its short persistence, when young larvae are present make 2 applications about 10 days apart, thoroughly spraying the lower foliage.
Other foliar sprays for leaf beetles include azadirachtin and spinosad. Avoid foliar application of carbamates (e.g., carbaryl), organophosphates (acephate, malathion), and pyrethroids (permethrin and other "-thrin" insecticides) because of their negative impact on bees and natural enemies and potential for drift and movement to contaminate surface waters. Translocated (systemic) neonicotinoids (e.g., imidacloprid) can be sprayed on foliage or drenched (watered) or injected into soil during fall or spring to provide season-long control.
For more information, see Chrysolina quadrigemina (Coleoptera: Chrysomelidae) Klamathweed Beetle from Cornell University and The Role of Chrysolina gemellata [=C. quadrigemina] in the Biological Control of Klamath Weed from the U.S. Department of Agriculture and University of California.
Adapted from Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).