The effect of temperature on the photoperiodic 'clock' and 'counter' of a Scottish clone of the vetch aphid, Megoura viciae

Photoperiodic response curves were determined for a Scottish clone of the v etch aphid, Megoura viciae Buckton, at three temperatures: 12.5, 15, and 17 .5 degrees C. Critical night lengths (CNLs) for ovipara (sexual female) ind uction were 6 h, 7 h and 8 h, respectively. High incidences of ovipara prod uction were observed in all night lengths longer than the CNL including con tinuous darkness (DD), as well as in continuous light (LL) at 12.5 and 15 d egrees C. At the same three temperatures, the number of long- or short-nigh t cycles required for half of the experimental aphids to be ovipara produce rs (i.e. the required day number, RDN) was determined. The RDN for long-nig ht cycles (LD12:12) could not be determined at 12.5 degrees C, but was temp erature compensated between 15 and 17.5 degrees C. The RDN for short-night cycles (LD20:4) could not be determined at any temperature. However, as ind uction of oviparae was always 100% in 12.5 degrees C, 94-100% in 15 degrees C and dropped from 100% to between 47 and 71% in 17.5 degrees C, it seems that short-night accumulation was temperature dependent. When fourth-stadiu m larvae were transferred from LD20:4 at 20 degrees C to the same light-dar k cycle at 15 degrees C, the aphids, when adult, switched to the production of oviparae after about 4 weeks. First-born progeny kept in LD20:4 and 15 degrees C switched to the production of oviparae about 7 days after the mou lt to adult. Thus, the photoperiodic response can be directly affected by t emperature, irrespective of photoperiod. Model-generated response curves us ing the 'double circadian oscillator model' for photoperiodic time measurem ent (Vaz Nunes, M., 1998. A double circadian oscillator model for quantitat ive photoperiodic time measurement in insects and mites. Journal of Theoret ical Biology 194, 299-311) closely resembled the observations. Differences between these data and the results of previous experiments with an English clone of M. viciae could be accounted for by differences in the photoperiod ic clocks (damping rate and period) as well as the photoperiodic counters. (C) 2000 Elsevier Science Ltd. All rights reserved.