The same photoperiodic clock may control induction and maintenance of diapause 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 maintenance (in the adult female) are under photoperiodic control. The question of whether or not the same photoperiodic clock is involved in both photoperiodic reactions was investigated in eight strains of the spider 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 breaks on the capacity of long nights to maintain diapause, and comparison 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 the critical daylength for diapause maintenance, measured at the moment of saturation of the response to long daylengths, was identical to the critical daylength for diapause induction. However, the critical daylength for diapause maintenance appeared to be labile; it shifted gradually to shorter values as the mites were kept in the cold for a longer period of time, or were kept at a higher temperature for a progressively longer period of time after their stay in the cold room. This seems to reflect a gradual loss of photoperiodic control of diapause maintenance as diapause development proceeds. Photoperiods close to the critical daylength appeared to be less strong with regard to diapause maintenance than shorter daylengths. Quantitative differences in the "strength" of different daylengths were found in all strains investigated. 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 sensitivity to the light breaks occurred at the same position in the night for diapause induction and diapause maintenance. The many similarities found lead to the conclusion that most probably the same photoperiodic clock mechanism is involved in both diapause induction and diapause maintenance in T. urticae.