The Ecology and Impact of the Broom Gall Mite (Aceria genistae) On Its Host Plant Scotch Broom (Cytisus scoparius)

Abstract

Scotch Broom (Cytisus scoparius) is a deciduous shrub, with green stems, small alternate leaves and conspicuous yellow flowers. The species has been widely introduced and is a major invasive weed in New Zealand, Australia and USA. Broom has a broad environmental niche and in New Zealand this species is now affecting agriculture, plantation forestry, railway lines maintenance and conservation areas, and is a priority for biological control. The broom gall mite Eriophyidae (Aceria genistae Nalepa) was released in 2008 as one of several biological control agents for broom. These mites cause death of the plants by forming “cancer” like stem-galls. However, there are other predatory mites on broom and bacteria and fungi appear to be associated with the galls. This species complex could potentially facilitate or moderate the effect of the gall mites as a biocontrol agent. The major predatory mites found on broom are Phytoseiidae (Typhlodromus caudiglan Schuster), and Stigmaeidae (Zetzellia māori González-Rodríguez). Two other prominent fungivorous groups that are abundant on broom are Tydeidae (Tydeus lambi Baker) and Tarsonemus species (Tarsonemidae). A manipulative experiment was set up to determine the impact of the broom gall mite Aceria genistae on its host plant Scotch broom and to investigate the effects of manipulating relevant predators, competitors and pathogens associated with Aceria genistae. This was achieved in the field using different pesticides and fungicides. The experiment was set up at Leslie Hills Station, North Canterbury (42°38'20.10"S, 172°46'49.62"E). Pre-treatment analysis of broom plant biomass showed no significant difference between treatments whereas post treatment results showed significant patterns over the first five months of data collection. Post treatment results showed that plants with Aceria genistae had highest growth suppression and highest plant mortality. Unfortunately penetrants associated with some treatments also appeared to suppress mite populations generally, weakening the impact of the targeted treatments on different groups of mites and our ability to detect impacts on broom. The experiment needs to be evaluated over a longer time period to fully understand the impact of gall mites on broom and the influence of interactions with other mite groups. Further investigations are required to grow and sustain populations of mites in the laboratory.