Invasion of the gall mite Aceria genistae (Acari: Eriophyidae), a natural enemy of the invasive weed Cytisus scoparius,into California,U.S.A. and predictions for climate suitability in other regions using ecological niche modelling

Scotch broom (Cytisus scoparius (L.) Link) is a European shrub that has naturalised in several countries worldwide and is recognised as an invasive weed in much of western North America. The mite Aceria genistae (Nalepa) is a coevolved, gall-inducing herbivore associated with Scotch broom in its native range and has been intentionally introduced as a classical weed biological control agent of C. scoparius in Australia and New Zealand. An adventive, never intentionally introduced, population of A. genistae was discovered in Washington and Oregon, U.S.A. in 2005. Surveys for A. genistae in California resulted in the discovery of the gall mite in 11 counties, with a widely scattered distribution. Molecular and morphological assessments confirm the mites collected from galls in California are A. genistae. Whether natural or anthropogenic, the estimated rate of long range dispersal for A. genistae from Washington or Oregon to California ranges from 39 to 62?km/yr. Niche model predictions indicate that A. genistae will continue to expand its distribution throughout much of the Scotch broom-invaded lands of California but areas supporting the weed in the Eastern U.S.A. appear less suitable. Modelling evidence also indicates that portions of Chile and Argentina are suitable for colonisation by A. genistae, also suggesting that expansion of the mite is possible in areas of Tasmania, southeastern Australia, and New Zealand where the mite was released. The environmental safety of A. genistae in relation to non-target plants and the influence of herbivory on Scotch broom fitness are discussed.