Abstract:
Cytisus scoparius, Scotch broom, a leguminous shrub native to Europe, is a noxious weed in several countries, including New Zealand. With a broad environmental niche, Scotch broom invades pastures, forestry plantations and conservation areas. It is highly unpalatable to livestock and can out-compete rare and endangered native herbs and shrub species of grassland communities and other areas of conservation value. The success of invasive plant species in their introduced habitats is largely attributed to the absence of natural enemies found in their native range. To halt the spread of Scotch broom in New Zealand, Aceria genistae, an eriophyid gall mite, was released as a biological control agent in 2007. Although, the gall mite has established well at the four original release sites in Canterbury, it is not yet apparent how effective it is at reducing target weed populations and the impact of the ecological interactions that it forms with other predatory, fungivorous and herbivorous mites. Therefore, this project aimed to: (1) continue a long-term manipulative experiment monitoring the impact of gall mites on growth, mortality and fecundity of broom by manipulating several mite guilds; (2) evaluate indirect effects of A. genistae on the native gall mite Aceria carmichaeliae via augmentation of predatory mites; and (3) understand effects of the type of diet on development, reproduction and survival of a predaceous and a fungivorous mite associated with A. genistae. The manipulative experiment showed that A. genistae is a highly effective biocontrol agent against Scotch broom as it reduced the plants’ growth rate and fecundity and increased rate of mortality. However, due to unsuitable abiotic factors and inappropriately timed introduction, the native A. carmichaelia failed to establish on the experimental plants so indirect effects of A. genistae could not be tested. Furthermore, the fungivorous and the predaceous mites failed to develop under the diets provided in laboratory experiments. This study aimed to improve understanding of A. genistae efficiency as a biological control agent for reducing abundance and distribution of Scotch broom as well as providing information on potential indirect interactions the gall mite may form with the native flora and fauna of New Zealand upon large scale release.
