Laboratory evaluation of avermectin as a selective acaricide for use withMetaseiulus occidentalis (Nesbitt) (acarina: Phytoseiidae)

The suggestion that adding a light oil to avermectin B₁ would increase the toxicity of avermectin to spider mites and reduce its effect on predaceous mites was tested in laboratory trials withTetranychus urticae Koch andMetaseiulus occidentalis (Nesbitt) on almond and bean foliage. No differences were found in the toxicity of avermectin + oil vs. avermectin alone at the doses tested forT. urticae; all (0.025, 0.5, 1, and 5 ppm) were highly toxic. Mortality ofM. 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 toT. urticae adults butM. 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 toT. 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, whenM. 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 toT. urticae than toM. 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.