Weevils turned loose against thistle Weevils turned loose against thistle (August 03, 2005)

They come from Greece and they look weird, but high hopes are riding on the 4,000 hairy weevils that have been turned loose in five open space preserves in the Peninsula hills.

The critters are less that one-quarter of an inch long, with long snouts, brown and gray stripes, and hairy bodies. And they love to munch the non-native yellow star thistle that plagues lands of the Midpeninsula Regional Open Space District.

This is a bad weed, said Cindy Roessler, the district's resource management specialist. Not only does the spike thistle deplete soil moisture and drive out desirable forage plants, it covers large areas with spiny seed heads. "These make hiking uncomfortable and can cause chewing disease in horses," she said.

The hairy munchkin attacks the thistle with a one-two punch, Ms. Roessler explained. Adults eat developing buds, while their babies eat miniature seeds from inside.

The double whammy happens when a fertile female chews a hole in the buds just before they flower, and then lays an egg in a hole. The egg hatches in three days; the larvae feed for two weeks, then pupate inside the flower bud, Ms. Roessler said. This is not good for the thistle, particularly since the adults continue feeding outside.

The district first tested the hairy weevils in 1997 at Fremont Older and Sierra Azul preserves, and in 2004 at Monte Bello and Russian Ridge preserves. Follow-up monitoring showed that 40 percent to 90 percent of seed heads were damaged, Ms. Roessler noted.

So, while this attack will set back the spread of the thistles, it will not eliminate them entirely. Therefore, the district is using other methods to get rid of the thistle, including mowing, cutting, pulling, prescribed burning, and selective application of herbicides.

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