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The suggestion that adding a light oil to avermectin BI would increase the toxicity of avermectin to spider mites and reduce its effect on predaceous mites was tested in laboratory trials with Tetranychus urticae Koch and Metaseiulus occidentalis (Nesbitt) on almond and bean foliage. No differences were found in the toxicity of avermectin + oil vs. avermectin alone at the doses tested for T. urticae; all (0.025, 0.5, 1, and 5 ppm) were highly toxic. Mortality of M. occidentalis females and larvae was not different on avermectin + oil vs. avermectin alone, but females produced more progeny on the avermectin + oil-treated foliage. At doses of 0.5 to 5 ppm, avermectin was sufficiently toxic to deplete predator populations in the field. Development of predator larvae on avermectin + oil and on avermectin alone was not different. Avermectin + oil on almond foliage aged outdoors was highly toxic after 96 h to T. urticae adults but M. occidentalis larvae survived well on residues by 96 h.M. occidentalis female survival and productivity were not different from the controls by 48 h. Hence a predator mite population might recover through larvae hatching onto residues. Avermectin + oil (3 ppm) residue on bean foliage held outdoors was still highly toxic to T. urticae after 33 days. In contrast, M. occidentalis females and larvae survived well on 48-to 96-hour-old residues. Neither predators nor spider mites placed on treated foliage (3 ppm) were able to reach untreated foliage in tests using bean plant seedlings with one leaf sprayed and one left unsprayed.Furthermore, when M. occidentalis females were exposed to 3 ppm avermectin for 300 s or longer, mortality was significant and the fecundity of females that had been exposed for as few as 30 s was reduced significantly. Thus, while avermectin is significantly more toxic to T. urticae than to M. occidentalis, its value as a selective acaricide will depend upon learning to use it at rates that will allow the retention of sufficient prey so that surviving predators can persist. Based on these laboratory tests, such selective doses are likely to lie below 1 ppm and can best be determined in field trials.

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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 es 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 pre-Continued inside back cover

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