THE SAME PHOTOPERIODIC CLOCK MAY CONTROL INDUCTION AND MAINTENANCE OFDIAPAUSE IN THE SPIDER-MITE TETRANCHUS-URTICAE

In the spider mite Tetranychus urticae, both diapause induction (which takes place during the larval and nymphal stages) and diapause mainte nance (in the adult female) are under photoperiodic control. The quest ion of whether or not the same photoperiodic clock is involved in both photoperiodic reactions was investigated in eight strains of the spid er mite, originating from different localities in Europe. The methods employed consisted of (1) determination of the relative importance of the photophase and scotophase in the two photoperiodic reactions; (2) comparison of photoperiodic response curves for diapause induction and diapause maintenance; and (3) determination of the effect of light br eaks on the capacity of long nights to maintain diapause, and comparis on with the effect of light breaks in diapause induction experiments. The scotophase appeared to be much more important than the photophase for both diapause induction and diapause maintenance. In all strains t he critical daylength for diapause maintenance, measured at the moment of saturation of the response to long daylengths, was identical to th e critical daylength for diapause induction. However, the critical day length for diapause maintenance appeared to be labile; it shifted grad ually to shorter values as the mites were kept in the cold for a longe r period of time, or were kept at a higher temperature for a progressi vely longer period of time after their stay in the cold room. This see ms to reflect a gradual loss of photoperiodic control of diapause main tenance as diapause development proceeds. Photoperiods close to the cr itical daylength appeared to be less strong with regard to diapause ma intenance than shorter daylengths. Quantitative differences in the ''s trength'' of different daylengths were found in all strains investigat ed. Interruption of the night by short pulses of light revealed either one or two peaks of sensitivity in the night, or one broad ''trough'' where the two peaks had merged. However, in each case maximal sensiti vity to the light breaks occurred at the same position in the night fo r diapause induction and diapause maintenance. The many similarities f ound lead to the conclusion that most probably the same photoperiodic clock mechanism is involved in both diapause induction and diapause ma intenance in T. urticae.