Weeds

Biocontrol of Yellow Starthistle

One reason exotics can be so invasive and destructive of our native vegetation is that the predators that evolved with the plant in its natural habitat did not migrate with the seeds and therefore the "escaped exotic" now has no natural control mechanism. As we watch broom, pampas grass, teasel, starthistle and other invasives increasingly crowd out our native vegetation we recognize now that these are here to stay. We spend thousands of hours attempting to control them, and while we may do so in certain limited areas, there is no doubt that we cannot be everywhere, and that we are losing the battle far more often than we win.

The August 4 1993 issue of AgAlert has an interesting article on current experiments in the control of starthistle with the one-quarter inch-long snout-nosed beetle, Larinus curtus, imported from Greece. The Larinus larvae eat the seeds of the Mediterranean native yellow starthistle (Centaurea solstitialis) which probably came into California accidentally in a load of grain seeds. Scientists identified the first specimen in the state in 1869.

The biological control program for yellow starthistle began in 1985 when the U.S. Department of Agriculture's Agricultural Research Service (ARS) imported and released the first batch of a seedhead weevil, Bangersternus orientalis. Since then, they have also released a hairy weevil (Eustenopus villosus), a gallfly (Erophora sirunaseva) and a peacock fly (Chaetorellia australis).

Of the four, all but the peacock fly appear to have become established. The peacock fly prefers slightly moister habitat and requires the bachelor button in addition to the yellow starthistle to reproduce. (Bachelor button is also a Centaurea).

Researchers believe the best way to control yellow starthistle is to import a platoon of biological control agents that will feed on different parts of the plant. ARS research botanist Charles Turner says, "In Italy and Greece, there are more than 40 species that feed on yellow starthistle and keep it under control. In those countries, you don't see large explosions of yellow starthistle."

Before importing a biological control agent, however, researchers must conduct extensive tests to ensure it won't feed on other plants. In the case of yellow starthistle feeders, they have to make sure the insect won't move into commercial crops such as safflower and artichokes that are closely related.

The Larinus weevil has been released near Ione in Amador County and near the Sutter Buttes in Sutter County. If they thrive and multiply they will be able to be spread further afield. One larva can destroy all the seeds in a flower head, which can number as high as 80. One starthistle plant can produce as many as 10,000 seeds. This year's release was only 200 weevils. Researchers are continuing to search for insects that feed on yellow starthistle leaves, stems and roots.

For a review of the problems posed by exotic plant species, see "Botanical barbarians," by Robert Devine, in Sierra, January 1994, page 52.