Pear psylla,Cacopsylla pyricola(Foerster), overwinters as an adult in reproductive diapause that is initiated and maintained by short photoperiod. In autumn, females have immature ovaries and few are mated. Coincident with warm field temperatures in late winter, their ovaries mature and most of them become mated. The mean number of spermatophores per female varied from 5.3 to 16.5 (range, 0–63) among populations from three orchards sampled in the spring. Newly emerged laboratory-reared winterform (diapause) males have active sperm in the testes and seminal vesicles. In autumn, field-collected winterform males have sperm in the seminal vesicles, but the rate of insemination of females is very low if males are held in the laboratory under a short photoperiod of 10:14 (L:D). The rate of insemination increases greatly after such males are conditioned at a long photoperiod of 16:8 (L:D) for about 10 d. As winter progresses, there is a decrease in the duration of the conditioning required to permit mating. The ability of males to mate with females appears to be under control of the diapause syndrome.