Spring colonisation of orchards by Anthonomus pomorum from adjacent forestborders

The early-season dispersal of the overwintered apple blossom weevil, Anthon omus pomorum (L.) (Coleoptera: Curculionidae), is a crucial stage in the co lonisation of dwarf apple orchards adjacent to forests. We have conducted r elease-recapture studies with 1700 to 4000 marked weevils at two orchard si tes in Switzerland over 2 years to characterise the spatial and temporal pa ttern of the dispersal process. The dispersal and colonisation of orchards in spring by overwintered weevils is dependent upon the prevailing temperat ure. An orientated dispersal from the forest border to the centre of the orchard was observed consistently, irrespective of the angle of the apple tree row s with respect to the forest border or of climatic conditions. The average dispersal distance of the weevils was 19 m. Approximately one third of the weevil population remained on the first tree encountered, the remainder of the population moved over short distances mainly along the tree rows. This dispersal pattern led to a strong 'edge effect' with higher numbers of weev ils occurring at the edges adjoining the forests as compared to the centre of orchards. The relevance of these findings to population dynamics and man agement of the pest is discussed.