Gorse
By Eva Varga, Coos SWCD EQIP Outreach Coordinator
Agriculture, America's biggest industry and largest employer, is constantly under threat of attack from foreign pests and diseases that, if introduced here, could cause costly damage to U.S. crops and livestock. Of particular concern is the potential introduction of non-indigenous weed species that could upset the balance among native plant species within natural and agricultural ecosystems in the United States. Because, in general, newly introduced foreign weed species have little or no competition from native plant species or no naturally occurring enemies, the weeds have the potential to multiply and quickly spread, displacing native plant and animal life.
Gorse is an exotic invader from Europe, originally introduced as an ornamental, it adapts to regions of mild winters and sandy or gravelly soils with abundant moisture. It is a spiny evergreen shrub in the pea family and is dense and stiff, forming impenetrable thickets. Its erect angular stems have spreading branches ending in thorns. Green leaves take the form of branching spines. Flowers are yellow and shaped like pea-blossoms, clustered near the ends of the branches. They are solitary or with two to three grouped in axils of spines on the preceding year's growth. Fruit pods (legumes) resemble pea pods that burst expelling seeds. Roots are very extensive, with woody crowns and nitrogen fixing nodules. Gorse resembles Scotch broom.
Vigorous stands grow outward, crowding out all other vegetation. It forms a center of dry dead vegetation. This, in combination with the oil content of the plant, presents a major fire hazard. In 1914, a fire destroyed the waterfront business district of Bandon. In 1936, the town was burned to the ground; 14 people died and only 16 buildings remained unburned. Each disaster was fueled by extensive infestations of gorse.
Biological control offers a long-term solution to the gorse problem and the chance of returning land to sustainable and productive use through an environment friendly control system. The possibility of a biological control for gorse is not new and began with the introduction of the gorse seed weevil (Apion ulicis or Exapion ulicis) in 1956. Although now widespread it has had no appreciable effect on gorse as it only attacks the spring crop of seeds, with the autumn crop being completely unaffected.
Several other biological control agents that attack gorse in their native environment of Europe have since been introduced. These include the gorse spider mite (Tetranychus lintearius Dufour), which is the most damaging agent on gorse in Europe. Mites are more closely related to spiders than to insects. They have sucking mouth parts and feed by piercing the cell wall and extracting cell contents. Gorse foliage becomes a bleached yellowish-white color, and growth is reduced where mites have been feeding.
Gorse spider mites range in size from about 0.4-0.6 mm in body length (smaller than a pin head). Immature mites have green abdomens with mouthparts and legs ranging from orange to yellow. Upon maturing, the body reddens often with a dark spot on either side of the abdomen. Gorse spider mites form discrete, long lived colonies that live in a tent-like white web and move around the host plant en masse feeding and web spinning as they go.
It is expected that the gorse spider mite will be one of several agents that will be required to collectively reduce the vigor of gorse. Biological control will not eradicate gorse but it is hoped that the combined impact of multiple control methods could significantly reduce plant vigor, making it more susceptible to grazing, weather stresses and herbicides and thereby enabling beneficial plant species to compete with it more readily.
