Agent: Spider mite Plant species attacked: Gorse Tetranychus lintearius Ulex europaeus
Impact on target plant: Mites damage the plant by sucking the juices from the spiny leaves, causing yellowing and desiccation. Heavily attacked plants produced 90% fewer flowers in the following year.
Collection and release: Collect infested plant material with mites, inoculate by tossing infested material into uninfested bushes. Ten infested branches per release are usually adequate. Avoid releasing mites from sites with predatory mites.
Distribution: The mite has been released and is established in 6 counties.
History: The gorse spider mite Tetranychus lintearius was released in 1994. It is the first spider mite introduced into the U.S. as a biological control agent for a weed. It is widely established, though rare, at gorse sites in Oregon. Mites were imported from New Zealand, and represent populations collected from Great Britain, Portugal, and Spain. An intensive interagency redistribution project was conducted in 1996. Over 270 releases of mites were redistributed throughout S Oregon coastal infestations. The mites make extensive webbing, which can enshroud entire plants.
Cooperative studies to monitor impacts of the mite on gorse and predatory mites that attack the gorse spider mite were conducted in conjunction with USFS and OSU. Several species of predatory mites have been found to attack the gorse spider mite, the most serious being Phytoseiulus persimilis, which was first found in the Bandon area (Pratt et al. 2002 and 2003). This predator reduced the gorse mite impact by more than 90% during 1998. Very few colonies were found after 1999, and most were infested with the predator. The predatory mite is not known to be established elsewhere in Oregon outside of greenhouses.
Also, a predatory ladybird beetle (Coccinellidae), Stethorus punctillum, was also found attacking gorse spider mites in the Sutherlin and Florence areas. A species of Scymnus was also found at Baker Beach, but its impact is unknown. The effect on mite populations has not been measured, but it appears to deplete mite colonies during the summer months.
Oregon Department of Agriculture Noxious Weed Control Program Photos by Eric Coombs, Oregon 635 Capitol Street NE Salem, OR 97301 Department of Agriculture, 503-986-4621 www.oregon.gov/ODA/programs/Weeds/Pages/Default.aspx Bugwood.org March 2016