Scotch Broom (Sarothamnus scoparius (L.) Wimmer) and Its Insect Fauna Introduced into the Pacific Northwest of America

Abstract

The fauna of Sarothamnuts scoparius, Scotch broom, is a rich one and has been studied at Silwood Park, Sunninghill, Berks., by Professor 0. W. Richards, Dr J. P. Dempster, myself and a number of other people for the last 10 years. At Silwood the species supports thirty-five phytophagous species and a large complex of their parasites and predators. In the British Isles broom is a common shrub on disturbed sandy soils, while its European distribution stretches from southern Scandinavia to Spain (Clapham, Tutin & Warburg 1957). It is also abundant on the hillsides around Rome and occurs in Madeira and Teneriffe. Eastward in Europe it is not found beyond Poland and Hungary. On the Pacific coast of North America it has been introduced into California, Oregon, Washington and British Columbia (Fernald 1950; Gieckey 1957). It has also been introduced into the eastern United States along the Atlantic coast from Nova Scotia to Virginia (Gleason 1952), but, in contrast to its flourishing growth in the Pacific Northwest, it is a poor competitor with the local lush vegetation of the eastern states (Frick 1962). In the Pacific Northwest, and in California, it is considered a pest in range and forest lands, since it competes with seedling trees, hinders reforestation and increases fire hazard among the older trees (Holloway 1964a). In parts of California it is a weed of sufficient importance to warrant the introduction of some of its insect enemies (Frick 1964; Parker 1964). In California broom occurs in three of the five main biotic provinces cited by Storer & Usitiger (1963), namely: (1) the north-west humid coast belt, which in Merriam's classification (in Jepson 1957) corresponds to the redwood (Sequoia sempervirens) transition life zone; (2) the Sierran Foothills, or digger pine (Pinus sabiniana)-chaparral belt (Merriam's Upper Sonoran zone); and (3) the lower part of the yellow pine (P. ponderosa) belt of the Sierra Nevada. In British Columbia, broom occurs in three of the fifteen biotic areas described by Cowan & Guiguet (1960), i.e.: (1) the Coast forest biotic area, where the climax forest is dominated by the western hemlock (Tsuga heterophylla), Sitka spruce (Picea sitchensis), and broadleaf maple (Acer macrophyllum); (2) the Puget Sound lowlands biotic area, where most deciduous trees are secondary; and (3) the Gulf Island biotic area, which comprises the islands of Georgia Strait and a narrow strip along the south-eastern coast of Vancouver Island; Garry oak (Quercus garryana) and madrono (Arbutus menziesii) constitute the climax type.