PHOTOPERIODIC INDUCTION OF DIAPAUSE IN THE SPIDER-MITE TETRANYCHUS-URTICAE - QUALITATIVE OR QUANTITATIVE TIME MEASUREMENT

Insects and mites may measure photoperiods either by classifying them as long or short relative to a critical value (qualitative time measur ement) or by using the absolute value (quantitative time measurement). The spider mite Tetranychus urticae is thought to use a qualitative m echanism of time measurement. In this paper we present the results of experiments with an inbred line of the spider mite (to keep genetic va riation in photoperiodic responses small), to test whether quantitativ e aspects also play a role. Differences in diapause incidence in diffe rent long-night photoperiods at different temperatures may be an indic ation of quantitative responses to photoperiod. The effect of temperat ure on the photoperiodic response curve was studied at 16 degrees C, 1 9 degrees C and 22 degrees C. The response curves appeared to be simil ar at 16 degrees C and 19 degrees C, with a critical nightlength betwe en 10 and 11 h. At 22 degrees C, diapause induction was less than 100% in all long-night regimens and the critical nightlength had shifted t o 12 h. Maximum diapause induction (93%) occurred in a light-dark cycl e with a 16 h dark phase (LD 8:16 h). Diapause induction was lowest in long-night photoperiods with dark phases of 20 h and longer. The numb er of light-dark cycles needed for 50% diapause induction at 19 degree s C varied between 12.1 and 14.7 for LD 6:18 h, between 10.9 and 12.5 for LD 8:16 h, between 10.6 and 11.6 for LD 10:14 h, and between 10.1 and 10.7 for LD 12:12 h. Independent of the light-dark regimen, diapau se induction took place in some individuals after receiving 8 cycles a nd virtually all individuals entered diapause after 16 cycles. No effe ct was found of the photoperiodic treatment during prediapause develop ment (LD 6:18 h, LD 8:16 h, LD 10:14 h, LD 12:12 h) on diapause durati on. The average diapause duration at LD 10:14 h and 19 degrees C was 6 1 days over all four treatments. We explained the results by hypothesi sing that nightlengths are assessed qualitatively and that the photope riodic clock operates more accurately near the critical nightlength.