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Three species of orchard mites and their predators on apple trees in the southern Okanagan valley of British Columbia, and the effect of three insecticides on this complex Chant, Donald Alfred

Abstract

The behaviour of the orchard mites Bryobia pretiosa (Koch) and Metatetranychus ulmi (Koch) and four species of predators was investigated both on unsprayed and on sprayed apple trees. On unsprayed trees the host mites were found to be most abundant on the east sides; no consistent directional preference was found for the predacious species. Small apple leaves supported the same number of M. ulmi per unit area as large leaves, but more mites of the genus Tetranychus. On unsprayed trees a negative correlation between the total number of predators and the number of host mites was recorded. In abandoned orchards the predators were capable of controlling the host mites and maintaining them at a level below that of economic significance. Each of the insecticides DDT, Parathion, and lead arsenate, was found to be very toxic to all species of predators recorded, and thus hindered or entirely prevented natural control of the host mites. In every instance lead arsenate and DDT produced an increase in the population of B. pretiosa, this effect being partially chemical and partially due to a lack of predators on the sprayed trees. Lead arsenate caused an increase in the population of M. ulmi whereas DDT reduced it to a slight extent. Under arid conditions Parathion caused an increase in the population of B.pretiosa; under less arid conditions the mite was eliminated. Parathion had no effect on M. ulmi in a commercial orchard while, in the same orchard, it caused a great increase in Tetranychus spp. The latter species were reduced by DDT in the commercial orchard.

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