After more than 20 years of study, the US Department of Agriculture has concluded that a tiny mite can be turned loose safely to contain Russian thistle — the tumbleweed. The Animal and Plant Health Inspection Service (APHIS) announced on 14 September that it will issue permits to release the non-native mite Aceria salsolae as a biological control throughout the Lower 48 states.

The 0.2-millimetre mites do not devour the plant; they insert needle-like mouthparts into its cells, stunting growth and hindering reproduction, and studies show they cannot sustain themselves on other plants, so native species are unlikely to be fed on. When they have finished with one thistle they climb to the top and wait for a gust of wind. Many probably die in transit, APHIS says, but the mites are likely to establish wherever Russian thistle grows. Like the thistle, they are native to Eurasia; they will not eradicate it, but should help control it where herbicides and tillage have largely failed.

Russian thistle reportedly arrived in the United States in a bag of Old World flax seed planted in Bon Homme County, South Dakota, in the 1870s and was the bane of Plains farmers before the century was out. It now infests 100 million acres in the West. Cattle graze the young plant in extreme drought, but its high salt content causes diarrhoea; growing plants harbour crop pests such as the beet leafhopper, and dead, dried tumbleweeds clog irrigation canals and fuel wildfires.

APHIS first proposed the release in 2009 and received only three comments — one from a USDA entomologist who noted that mites had survived up to five weeks on other plants in experiments. More years of study followed until the agency concluded last year that those plants cannot sustain mite populations; the renewed proposal again drew three comments, all in favour.

The California Department of Food and Agriculture said it will begin rearing and releasing the mites immediately. "We expect to receive requests for this agent from ranchers, farmers and the general public," the department said.

Source: Capital Press