Differences in premating behavior patterns and timing, both within and between populations, were observed among reciprocal crosses of five populations of the western predatory mite, Metaseiulus occidentalis (Nesbitt). One population that was subjected to three selections for enhanced premating isolation showed no selection response.

Postmating isolation in five of the eight pairs of reciprocal crosses of M. occidentalis colonies resulted in the deposition of shriveled eggs and reduced numbers of apparently normal eggs. Reciprocal crosses usually exhibited different degrees of incompatibility; no apparent pattern with regard to geographic origin of the colonies was found in the degree of postmating isolation.

The potential impact of postmating isolation on population dynamics was evaluated in reciprocal crosses of two populations using life table techniques. One colony was a strain that had been artificially selected in the laboratory for resistance to permethrin (Immature Selection-38), the other a population collected from a California pear orchard (McCall Pears). In the Immature Selection female (IS) X McCall Pear (MP) male cross, IS females produced fewer eggs than MP females in the reciprocal cross. Of the eggs deposited by the IS females mated to MP males, substantially more eggs shriveled than in the reciprocal cross. In addition, most of the surviving F₁ progeny in the IS female X MP male cross were males. The net reproductive rate (R_o) of the MP female X IS male cross was 10.58, but only 0.92 for the reciprocal cross. The intrinsic rate of increase (r_m) of the MP female X IS male cross was 0.213 and zero for the reciprocal cross.

The wide geographic distribution of M. occidentalis throughout western North America enhances the likelihood that genetically distinct populations have developed. The data presented here and other data previously obtained on dispersal rates, discreteness of pesticide-resistant populations, and differences in diapause attributes of geographic strains indicate that M. occidentalis is subdivided into distinct populations, some of which are partially reproductively isolated from others through postmating isolating mechanisms. No clear pattern has emerged, however, that would allow us to predict which populations are completely compatible. Populations that are geographically distant may show little reproductive isolation, while others that are adjacent may be partially reproductively isolated, and differences may occur between reciprocal crosses. In the case of the permethrin-resistant strain (IS), the degree of reproductive isolation after release into orchards or vineyards that contain native M. occidentalis cannot be predicted. It would be desirable to have such reproductive isolation, because permethrin resistance is polygenically determined in this colony and outcrossing can result in loss of resistance. Thus, IS (permethrin-resistant) populations released into North American orchards or vineyards will have to be maintained as pure populations through selection with permethrin rather than through reproductive incompatibility because of our inability to predict whether postmating isolation will occur.