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

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Abstract
Scotch broom (Sarothamnus scoparius) has spread and become a weed. The main broom sites in California and in British Columbia were visited and the insect fauna was examined. Some of the introduced insect species that were found are new records for California or for British Columbia. Most of the introduced insects on broom are those which aestivate or hibernate in their immature stages under bark of young stems. They were probably transported with young host plants which were introduced either deliberately, or accidentally in ballast. Micro-peza lateralis and Minettia fasciata were most probably carried over a puparia in broom litter in ballast. The most abundant broom insects in California are the mirid Orthotylus concolor, the moth Leucoptera spartifoliella and the psyllid Arytaina spartiophila. The broom fauna of British Columbia is much richer and the most abundant insect was Orthotylus virescens. The populations of O. concolor in California and of O. virescens in British Columbia are enormous. Their expansion is probably associated with climatic conditions favorable to these species and with the absence of many natural enemies, notably the early hatching broom Miridae which are predacious as well as phytophagous. Samples from populations of broom Miridae from North America were compared with those from southern England. Eight characters were measured and subjected to the canonical variate analysis. The ranges of variation in samples of populations of O. virescens from southern England were as great as those between different geographical regions. O. concolor is a more plastic species, but although its populations may have been isolated for more than a hundred generations in the Sierran foothills, no major departures were found in the characters studied.