Effects of gamma radiation on the biology and population suppression of the two-spotted spider mite, Tetranychus urticae Koch

Abstract

Laboratory investigations were made in California on the sterilising doses of gamma -radiation from a 60Co source for Tetranychus urticae Koch, and control of the mite in strawberry fields was attempted in 1968 using the sterile-male technique. When mortalities were measured in the laboratory after 10 days, the LD50's were found to be 90 krad for adult males and 110 krad for adult females. Dosages of 3-3.2 krad were lethal to all one- and two-day-old eggs, but eggs became more resistant to radiation as they aged and nearly all 3- and 4-day-old eggs hatched after exposure to 11.5 krad. With dosages of 20 krad, male protonymphs, deutonymphs and adults were all rendered sterile, and eggs and larvae were killed. A sterilising dose did not alter the mating competitiveness or length of life of males, and a dose of 100 krad did not appear to cause inactivation of sperm [cf. RAE/A 53, p. 32]. A substerilising dose given to a male subsequently mated to an untreated female caused sterility in the F1 female progeny. The tests supported the belief that males undergo multiple matings. The eggs of about 10% of the females undergoing secondary matings were successfully fertilised by more than one male [cf. 55, 1951]. In two field tests, sterile males were released at rates of 9 and 19 sterile males/untreated male in the observed population, once only or at 1- or 2-weekly intervals for up to 13 weeks, and the numbers of mites/leaflet were counted at weekly or 2-weekly intervals. The numbers of mites on treated plants did not differ significantly from those on untreated ones, and the yield of fruit did not differ from that on untreated plants or on plants that had been subjected to chemical control.