ACCUMULATION OF PHOTOPERIODIC INFORMATION DURING DIAPAUSE DEVELOPMENT IN THE SPIDER-MITE TETRANYCHUS-URTICAE

Photoperiodic control of diapause development was studied in three strains of the spider mite, Tetranychus urticae, originating from different latitudes in Europe. Diapause was almost fully maintained by a long-night regime during the first month of diapause, but was terminated rapidly and synchronously by a short-night regime. Diapause termination in continuous light was almost as rapid as in short nights; in continuous darkness diapause termination proceeded much slower, probably revealing the ''spontaneous'' rate of diapause development of these mites. The effect on diapause termination of successive short-night cycles appeared to be accumulated in a way comparable to cycle summation during diapause induction. The threshold for diapause termination by short nights, expressed as the number of cycles required for 50% diapause termination, was found to be lower the more southern the origin of the strain of mites. However, considerable differences in the number of cycles required for diapause termination were also present within strains: some mites needed only 3-4 short nights to terminate diapause, whereas others needed more than 10. In contrast with short-night cycles the effect of long-night cycles was not accumulative. Intensification of diapause under the influence of long-night cycles during its early stages appeared not to take place. Long nights were capable of maintaining the state of diapause if experienced before short-night cycles, but not afterwards. Short and long nights seem to act independently from each other, the former accelerating diapause development (''activation''), the latter slowing down diapause development (''diapause maintenance''). No antagonistic effect of long and short nights was found on the photoperiodic maintenance of diapause, in the sense that the effect of short nights might be diminished or even reversed by long nights, and vice versa, as found in photoperiodic induction of diapause in T. urticae. Therefore diapause maintenance does not seem to involve a photoperiodic counter mechanism comparable with the counter involved in the photoperiodic induction of diapause in these mites.