Geographic variation of diapause intensity in the spider mite Tetranychus urticae

Abstract

Abstract. Eight strains of the spider mite Tetranychus urticae, originating from different localities in western and central Europe, with latitudes ranging from 40.5 to 60oN, displayed marked differences in the period of chilling at 4oC required for diapause termination under a diapause‐maintaining short‐day photoperiodic regime at 19oC, to which the mites were transferred after the cold period. The higher the latitude from which the strains originated the longer was the period of chilling required for diapause termination, suggesting the presence of a gradient in diapause intensity, diapause being deeper the more northern the origin of the strains.Two strains originating from higher altitudes appeared to have a much deeper diapause than expected from their latitudinal origin. In addition, these two mountain strains showed mutual differences in diapause intensity, notwithstanding the fact that they originated from similar latitudes and altitudes; local climatic conditions probably act as strong selective forces with regard to diapause depth.All strains appeared to be sensitive to photoperiod during the period of diapause development. Diapause was quickly completed by a long‐day photoperiod (LD 17:7 h), but was maintained by a short‐day photoperiod (LD 10:14h). However, even under the latter regime sensitivity to photoperiod gradually diminished and eventually disappeared, thus leading to ‘spontaneous’ termination of diapause. The length of the period of diapause development, as measured by the sensitivity to photoperiod of diapausing mites, varied between strains; it was shorter in the southern strains and longer in the northern strains. The results indicate great variation in diapause intensity between strains, which is probably genetically determined and may have adaptive significance for this widespread species.When young females which had just entered diapause were kept for ever longer periods of time under the diapause inducing short‐day regime at which they had been reared, before being transferred to the cold room, the duration of the period of chilling required for diapause termination was found to decrease proportionally in all three strains tested. These results suggest that intensification of diapause does not occur in T. urticae; diapause intensity seems to be highest at the beginning of diapause and to diminish gradually during diapause development.