Some biological effects of gamma radiation on the two-spotted mite, Tetranychus urticae Koch were determined. A substerilizing dose given to a male subsequently mated to a normal female caused sterility in the F₁ female progeny. LD50/10 values for adult males and females were established at 90 and 110 krads, respectively. Eggs were more radioresistant as they aged; 3,000 to 3,200 rads were lethal to all one- and two-day-old eggs, while most all three- and four-day-old eggs hatched after an exposure to 11,500 rads. Male protonymphs, deutonymphs, and adults were all rendered sterile, and eggs and larvae were killed with 20 krads. A dose of 100 krads did not appear to cause sperm inactivation in males. The tests supported the belief that males undergo multiple matings. The eggs of about 10 per cent of the females undergoing secondary matings were successfully fertilized by more than one male. A sterilizing dose for males did not detrimentally alter mating competitiveness or longevity.

Control of this species in strawberries was attempted using the sterile-male technique. Field release of both sexes caused plant damage due to additional feeding activity. An all-male colony was obtained from isolated virgin females, but not enough males could be reared, sterilized, and released to reduce the endemic field population.