The activity levels (AL) of males and females of four genetically manipulated strains and one wild strain of the phytoseiid Metaseiulus occidentalis (Nesbitt) were measured using a computerized video-tracking system. Such AL measurements offer the potential for a quantitative laboratory technique to evaluate the relative quality of strains. Adult females of a Permethrin-Organophosphorous-resistant (P-R) strain consistently had an average AL significantly lower than those of the Wild strain and of the three other laboratory strains. The low activity of the P-R strain is unlikely to be due to decreased strain quality associated with laboratory rearing procedures per se, because the three other long-established laboratory strains exibited AL averages comparable to the Wild strain. Adult males exibited no differences in AL. Possible reasons for the lower AL of the P-R strain are discussed.

Hungry adult females tested for AL exhibited three types of running patterns: edge, circle, and nonspecific. The Wild females ran mostly in circles, whereas the P-R females ran mostly near the edge. The males of the P-R and Wild strains did not run in circles, and displayed nearly equal proportions of the edge and nonspecific patterns. Pattern type appears to be related to rate of movement.

A genetic analysis of AL was conducted using the Wild and P-R strains. The AL measurements of reciprocal F1 and F2 females indicated that the high AL of the Wild strain is dominant over the low AL of the P-R strain. Attempts to repeat the F1 and F2 tests were unsuccessful because of a previously undetected, one-way mating incompatibility between the reciprocal crosses; however, the data indicate that activity level is a heritable trait.