Evidence of a high level of gene flow among apple trees in Tetranychus urticae.

The dispersal mechanism of the two-spotted spider mite Tetranychus urticae Koch (Acari: Tetranychidae) could affect predator-prey population dynamics and the spread of acaricide resistance. To investigate the propensity for spider mite migration in the field, the genetic structure of spider mite populations was studied in two apple orchards using five microsatellite markers. Adult female mites were collected from trees separated by approximately 10-24 m along a line covering a distance of about 100 m. The genetic data suggested that a high population density increased the migration rate among the breeding colonies within a single tree. Spatial autocorrelation analysis suggested a positive genetic structure in the first distance class within the two orchards, which might have been caused by crawling or short-distance aerial dispersal. Meanwhile, mites may also have a large-scale migration system that could cause a high level of gene flow and constrained isolation-by-distance or genetic clines within the approximately 100-m range of the study sites. Therefore, mites might aerially disperse over long distances on a scale of <100 m while also taking shorter trips among nearby trees within a distance of 10-24 m in the apple orchards.