The impact of the gorse spider mite, Tetranychus lintearius, on the growth and development of gorse, Ulex europaeus

A replicated field experiment was conducted in Tasmania, Australia to assess the impact of Tetranychus lintearius on the growth and reproductive capacity of gorse. The dry weight of gorse shoots collected from T. lintearius attacked bushes and miticide-treated controls were compared over a two and a half year period. Over this period, T. lintearius underwent approximately five generations and caused a significant reduction in dry matter production of approximately 36% compared to the miticide-treated control. The reduction could have been greater as this figure was calculated after control plants sustained damage by T. lintearius. Predicted mean shoot dry weights of control plants in the absence of T. lintearius damage were calculated by solving a regression equation on the proportion of non-infested shoots per plant and mean final shoot dry weight. Using this method, it is predicted that a reduction in dry matter production caused by T. lintearius would have been approximately 44% had no damage to controls occurred. Two specialist predators of Tetranychus spp., Phytoseiulus persimilis and Stethorus sp., were both found at the site within the first year of the trial. Tetranychus lintearius was not found to negatively affect the damage caused by an established gorse biocontrol agent, the seed feeding weevil, Exapion ulicis. The use of T. lintearius as a biological agent for gorse in Australia, the interaction between agents and the impact of predators are discussed.