Gorse battles with Spider Mite

contents_bannerGorse spider mites are literally sucking the life out of gorse bush and helping to control the spread of the hardy shrub around the Granville area of the Grey Valley.

“The gorse spider mite, (Tetranychus lintearius) builds its web on gorse branches and lives on the plants’ sap, slowing the vigorous growth of the plant”, says Ross Jackson, forestry planning supervisor, Timberlands West Coast Ltd.

Part of a joint biological control programme between Landcare Research and Timberlands West Coast Ltd, gorse spider mites were first introduced to the coastal area between Greymouth and Hokitika about 10 years ago. The result was unsuccessful due to the high rainfall. A second attempt during 1992 in the drier area of Granville resulted in significant initial spread before colonies dwindled.

“This summer’s warm, dry conditions have helped encourage vigorous growth in numbers and we’re now finding colonies in Caribou, Waiuta, Holmes Hill at Reefton and throughout the Grey Valley,” says Mr Jackson, “It’s bad news for the gorse but great for those of us who want to see a decrease in the plants growth.”

Gorse spider mites, along with gorse seed weevil are the two agents currently used in the battle against gorse. The spider mites are proving more successful than the seed weevil that, as the name suggests, eats the plant seeds during the flowering season. Spider mites drain the plant of its vigor and stunts its growth all year round.

“Attempts are being made to establish other gorse-devouring species such as thrip (a type of aphid), the gorse pod moth, which together with the gorse seed weevil will account for up to 99 percent of gorse seed, and two types of moth, soft shoot and hard shoot. All species are thoroughly tested by Landcare Research to ensure they are specific parasites of gorse before being introduced to gorse patches,” Mr Jackson says.

“Gorse spider mites are identified by a web which covers the entire gorse branch, the mites appear as tiny brown dots on the web. They seem to prefer younger gorse plants of perhaps two or three years old. When mite-covered gorse is transferred to other gorse patches, the mites disappear within two to three weeks – they disperse on the wind and establish in colonies across other patches of gorse in the vicinity,” he says.