Abstract
Polymorphism of four enzymatic loci has been examined in 27 populations of Tetranychus urticae in relation to their geographical distribution and to two ecological parameters: open field vs. greenhouse habitats, and species of the colonized host plant. Genetic differentiation was significantly correlated to geographical distance in both types of habitat. Mite density and distribution of infested plants appear to be important factors for the population structure of T. urticae. In open field, T. urticae specimens from citrus trees were genetically more similar to other 'citrus' samples collected in different localities than they were from mites collected in the same locality on other plant species.