Genetic structure among 18 west coast pear psylla populations: implications for the evolution of resistance.

Patterns of genetic variation within and among 18 pear psylla ( Cacopsylla pyricola [Foerster[) populations were estimated from isozyme analysis and described in relation to geographic distribution along the west coast of North America. Eight of 22 enzyme loci were polymorphic. Average population differentiation was modest ( F ST = 0.08; range 0.025–0.183) and was greatest at two esterase loci which varied in activity but not mobility. Variation was greater on a local level—i.e., within river drainages, than it was between regions. This contrasts with the geographic pattern in insecticide resistance levels which are more variable among regions than within them.