Biological Control of Yellow Starthistle in Santa Clara County
Yellow starthistle (Centauria solstitialis) is a spiny annual plant native to the Mediterranean Basin that is highly invasive on grasslands and other environments in California and other western states. As well as being a nuisance to people, the plant is poisonous to horses and deters grazing by livestock. In its native environment, yellow starthistle is held in check by numerous natural enemies, referred to as biological control agents. After undergoing host specificity and a myriad of other post entry quarantine studies, several of these biological control agents have been secured by the USDA Agricultural Research Service for dispersal in the western states as a means of controlling the weed. Since 1991, a cooperative effort between the USDA, the California Department of Food and Agriculture, and the Santa Clara County Agricultural Commissioner's Office, has placed three of the biological control agents within Santa Clara County 'nursery sites'. The purpose of the nursery sites is to establish stable populations of the biological control agents from which in-county redistributions can be made.
Distribution of Yellow Starthistle Biological Control Agents in Santa Clara County
Bangasternus orientalis -- (Bud Weevil) -- This weevil has one generation per year, and deposits eggs on the scale leaves immediately below the closed head buds of yellow starthistle. Larvae bore into the heads where they feed primarily on receptacle tissue. A single larva in a head can destroy 50 to 60 per cent of the seeds.
Urophora sirunaseva -- (Yellow Starthistle Seed Head Gall Fly) -- This fly has two generations per year in California. Eggs are deposited on intermediate, closed head buds. Woody galls form around the developing larvae and there is one larvae per gall. The formation of the woody galls has two effects: 1) Within a galled head, the space occupied by the galls essentially replaces seeds, thereby reducing the number of seeds produced, and 2) The energy expended by the plant to make the galls results in a lesser number of heads produced.
Eustenopus villosus (Hairy Weevil) -- This weevil has one generation per year. Eggs are inserted inside late, closed head buds. The larvae feed on receptacle tissue and are capable of destroying most to all of the potential seeds in a head. Unlike the other insects, adult feeding also has a major impact on the weed. Adults feed on early, closed head buds, in the process completely destroying them. Because of the combined effects of adult and larval feeding, this insect seems to be a very promising agent for biological control of yellow starthistle.