Galls formed by Aceria genistae (Acari: Eriophyidae) alter reproduction of the invasive weed Scotch broom (Cytisus scoparius) in the USA

ABSTRACT The Scotch broom gall mite, Aceria genistae, is an adventive herbivore of Cytisus scoparius (Scotch broom), an invasive non-native shrub in California, USA. Although feeding by these mites clearly induces localised tissue distortion, the effect of A. genistae on overall host vigour and reproduction has not been assessed. Here we compare biomass allocation across various plant organs between two groups, heavily or lightly galled plants, at three northern California sites. Most structural components, including plant height, stem biomass, and leaf biomass, were similar regardless of galling intensity. However, both canopy volume and seeds per plant were reduced in heavily versus lightly galled plants; seeds were reduced up to 80% among larger plants. Collectively, these data suggest A. genistae galls are produced at the cost of seed production. Aceria genistae is a relatively new arrival in California but is already causing measurable effects to plant performance, which are expected to increase over time as mite populations increase and its range expands. Intentionally redistributing this mite is currently illegal in the USA, and this research is part of a broader investigation to inform the biological control agent permit process.