Intraplant distribution of Acalitus essigi (Acari: Eriophyoidea) on blackberries (Rubus fruticosus agg.).

Specialised phytophagous arthropods often display high levels of specificity to particular sites on their host plant. In this paper we examine the occupation of microhabitats and aggregation patterns of the eriophyoid mite, Acalitus essigi (Hassan), on its host plant, European blackberry (Rubus fruticosus L. aggregate), a plant that undergoes significant seasonal changes in its morphology. A. essigi was found to be a refuge inhabiting species. It resided in bud and leaf axil microhabitats on both primocanes and fructocanes and also occupied berry and bract microhabitats on fructocanes. Population fluctuations within the different microhabitats were evident across seasons. From summer to winter, populations significantly declined in bract and leaf axil microhabitats, but significantly increased within bud microhabitats where overwintering took place as slowly reproducing colonies. Live fruit and young shoots were also identified as overwintering sites. A. essigi populations displayed an aggregated distribution both within and between individual blackberry canes. Within primocanes A. essigi were aggregated in the lower 20% of cane length. On fructocanes aggregation of A. essigi was in the lower 20% and especially in the upper 20% of cane length. In spring A. essigi was confirmed to emerge from bud overwintering sites and colonise shoots mainly in the lower third of the previous season's primocanes, suggesting limited dispersal away from overwintering sites. It is proposed that biotic factors such as tissue age, microhabitat morphology and limited ambulatory dispersal capabilities are responsible for the aggregation pattems of this mite.