MROSD deploys weevils to control yellow starthistle

The Midpeninsula Regional Open Space District received 6,500 hairy weevils that will be released by the District today and tomorrow on Skyline and Foothill-area open space preserves as part of an effort to control and reduce the spread of the non-native, invasive weed yellow starthistle and restore the natural values of the preserves. The hairy weevil, Eustenopus villosus, one of several insect species researched by the U.S. Department of Agriculture, has been an effective biocontrol agent in the control of yellow starthistle.

“By introducing the weevils, a natural enemy of yellow star thistle, the District reduces removal costs and provides a long-term, sustainable method of control,” said Lisa Infante, the District’s Resource Specialist. “In effect, introduction of the weevil is an effort to recreate the natural system of ecological checks and balances that prevent this non-native, invasive plant from eliminating desirable native plants and habitats in the open space preserves.”

Originally from Greece, the hairy weevil is 6-8 mm long, has a distinctive long snout with antennae, and an overall brown-gray striped color and hairs covering much of its body. The hairy weevil attacks with adult weevils feeding on developing buds and larvae feasting from within on immature seeds, effectively diminishing the yellow starthistle seed production. To lay eggs, fertile females chew a hole in the side of buds shortly before the flowers open and deposit an egg. The larvae hatch in about three days, and after about two weeks of feeding, the larvae pupate inside the flower bud. Adults continue to feed on late-developing flower buds and then overwinter in seedheads or ground duff.

The District first released hairy weevils in 1997, and conducted additional releases in 2004 and 2005. New to the project this year, the District is implementing two monitoring methods. One method will cover selected yellow starthistle plants with netting prior to the release of the new batch of weevils in order to examine how the recently released weevils are responding compared to the weevils that were released last year. District resource staff will monitor the plants, focusing on the flower buds to count the number of remaining seeds. Staff will also compare damage to the seed heads between the netted and un-netted plants. It is assumed that the netted plants contain weevils released in previous years.

In addition, resource staff will be collecting the soil near the base of the yellow starthistle plants to examine the seed bank and determine if the number of yellow starthistle seeds is decreasing as a result of the weevils’ efforts. Counting the number of remaining seeds from the treated and untreated yellow starthistle populations (those with and without weevils) will also provide information on the effectiveness of the weevils. The District will continue to monitor the weevil populations for several years, and also plans to release weevils again next year. 

Initial follow up monitoring from previous years revealed that 40-90 percent of the yellow starthistle seedheads were damaged. Although hairy weevils will not completely eliminate yellow starthistle, they can substantially reduce the amount of seed produced within three years and are a good tool for locations where other control methods are not immediately feasible. The District is also implementing other methods to control and eradicate yellow starthistle, including mowing, prescribed burning, cutting, pulling, and select herbicide application.