Photoperiodic induction of diapause in the spider mite Tetranychus urticae: Qualitative or quantitative time measurement?

Abstract

Insects and mites may measure photoperiods either by classifying them as long or short relative to a critical value (qualitative time measurement) or by using the absolute value (quantitative time measurement). The spider mite Tetranychus urticae is thought to use a qualitative mechanism of time measurement. In this paper we present the results of experiments with an inbred line of the spider mite (to keep genetic variation in photoperiodic responses small), to test whether quantitative aspects also play a role. Differences in diapause incidence in different long-night photoperiods at different temperatures may be an indication of quantitative responses to photoperiod. The effect of temperature on the photoperiodic response curve was studied at 16°C, 19°C and 22°C. The response curves appeared to be similar at 16°C and 19°C, with a critical nightlength between 10 and 11 h. At 22°C, diapause induction was less than 100% in all long-night regimens and the critical nightlength had shifted to 12 h. Maximum diapause induction (93%) occurred in a light-dark cycle with a 16 h dark phase CLD 8:16 h). Diapause induction was lowest in long-night photoperiods with dark phases of 20 h and longer. The number of light-dark cycles needed for 50% diapause induction at 19°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, diapause induction took place in some individuals after receiving 8 cycles and virtually all individuals entered diapause after 16 cycles. No effect was found of the photoperiodic treatment during prediapause development (LD 6:18 h, LD 8:16 h, LD 10:14 h, LD 12:12 h) on diapause duration. The average diapause duration at LD 10:14 h and 19°C was 61 days over all four treatments. We explained the results by hypothesising that nightlengths are assessed qualitatively and that the photoperiodic clock operates more accurately near the critical nightlength.