Response to photoperiod during diapause development in the spider mite Tetranychus urticae

To study the question whether photoperiodic time measurement in the spider mite Tetranychus urticae is based on a qualitative or quantitative principle, the duration of diapause development was determined in individual females at various constant photoperiods at 19 degrees C. Diapause duration at all four long-night treatments fluctuated around 64.5 days, varying from 62.2 at LD 12:12h to 66.4 at LD 10:14h. The within-treatment variation in diapause duration of the long-night groups appeared to be significantly correlated to the nightlength of the photoperiods used; the longer the nightlength, the higher the within-treatment variation. Frequency distributions of females completing diapause under the two regimes with nightlengths near the critical nightlength were skewed to the right. Mean diapause durations at these regimes, LD 13:11h and LD 14:10h, were 25.4 and 11.9 days, respectively. Mites completed diapause rapidly and synchronously under the three short-night photoperiods tested; within two weeks after transfer from cold storage at 4 degrees C to the diapause terminating regimes at 19 degrees C all females started reproduction. Mean diapause durations were 8.1, 6.4 and 6.5 days for the short-night treatments LD 15:9h, LD 17:7h and LD 19:5h, respectively. The coefficients of variation of diapause duration (variability within groups relative to the mean) of the short-night and the long-night groups varied from 18 to 42%; the coefficients of the two intermediate groups were 69and 81%. There was a clear difference in diapause duration between long-night and short-night groups, but no significant difference was present in this characteristic between different long-night groups on the one hand and only a small difference between different short-night groups on the other. These results support the hypothesis that photoperiodic time measurement in the spider mite is based on a qualitative principle; photoperiods are classified as either 'long' or 'short' in relation to a 'critical' photoperiod. However, around the critical nightlength, intermediate responses were observed which might hint at the quantitative nature of the underlying mechanism. Therefore, although most results are in agreement with the hypothesis of a qualitative mechanism, it cannot be excluded that photoperiodic time measurement in the spider mite is based on a quantitative principle.