Thursday, March 26, 2009

USDA plans to loose biological agents against invasive thistles
Mite and weevil come to the rescue of crops, livestock

Mateusz Perkowski
Capital Press

Thursday, March 26, 2009

Russian thistle with sharp prickles on the leaf margins.

Scientists at the USDA want to deploy highly target-specific weapons against two common Western weeds. The agency's Animal and Plant Health Inspection Service may unleash biological control agents against the Russian thistle and yellow starthistle, invasive species that afflict crop and livestock production throughout the West. The potential effects of these biological control efforts have been compiled by APHIS in an environmental assessment and recently released to the public. A species of blister mite that originates in Turkey, Uzbekistan and Greece - Aceria salsolae - has been identified as a natural foe of the Russian thistle, which is native to southwest Asia.

Since being introduced to South Dakota in the 1870s, Russian thistle - the tumbleweed - has spread throughout the Western U.S., where it grows in disturbed soils and arid regions. "The infestation of Russian thistle causes millions of dollars of damage by disrupting automobile traffic, clogging irrigation canals, piling up against fences and houses, and igniting and spreading wildfires," according to APHIS.

The plant is an iconic image of the old West, particularly when it detaches from the soil and tumbles across the landscape. That tumbling is how the plant disperses vast numbers of seeds over large areas, according to University of California's integrated pest management guidelines. Aside from directly competing with crops for moisture and nutrients, the species provides habitat for insect pests and plant pathogens, according to the IPM guidelines. Herbicides, grazing and tillage, among other efforts, haven't been able to effectively suppress the weed, which is why APHIS is looking for biological alternatives, according to the agency.

The blister mite under consideration by APHIS hides in crevices within the Russian thistle and feeds on the plant's cell contents, resulting in the death of its growing shoots. The mite stunts the thistle's development and hinders its ability to reproduce, but APHIS research has shown that it doesn't negatively affect non-target plant species. The weevil - Ceratapion basicorne - that APHIS has identified as a natural enemy of the yellow starthistle does minimal damage to related non-target species, but many of them are invasive themselves. Weevil larvae hatch on thistle leaves and burrow down the plant's stalk and into the root crown, hindering growth and seed production.

Yellow starthistle was brought to California in the mid-1800s from Chile, where it was initially introduced by Spaniards in the 17th century, according to the University of California's Weed Research and Information Center. Since then, the weed has spread throughout most of the U.S., but the greatest concentration is in California, Oregon, Washington and Idaho, according to APHIS. As with the Russian thistle, conventional weed control methods haven't been sufficient. The weed "displaces desirable plants in both natural and grazing areas," APHIS said. Cattle and other grazing livestock usually avoid the plant's prickly spines, degrading the quality of rangeland. If horses do eat yellow starthistle, toxins in the weed can cause ulcers, brain lesions and death. "In addition to rangeland, pastures and grasslands, yellow starthistle is the most important roadside weed problem in much of Central and Northern California and has, on occasion, caused problems in dryland cereals, orchards, vineyards, cultivated crops and wastelands," according to WRIC.