Laboratory and Field Studies on Orchard-Mite Resistance to Kelthane1

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Abstract
In September of 1958, a few apple growers in Washington reported the failure of Kelthane® (1,1-bis( p -chlorophenyl) 2,2,2.. trichloroethanol) in controlling the McDaniel mite, Tetranychus medanieli McGregor. During 1959, this problem occurred in many other apple orchards in north central Washington. When difficulties were first reported, field studies indicated that timing, application, and the amount of active material used were not responsible for the failures, but rather that the mites had developed a resistance. Several other chlorinated hydro-carbon acaricides also failed to control the mites. Laboratory studies confirmed the presence of Kelthane-resistant strains of the McDaniel mite at several locations. One strain from an orchard where control had been especially difficult showed a 200-fold increase in the LD-50 over a susceptible strain. Laboratory studies with strains of the two-spotted spider mite, Tetranychus telarius (L.), from different areas showed that they were about equally susceptible to Kelthane.

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