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 vetch aphid, Megoura viciae Buckton, at three temperatures: 12.5, 15, and 17.5°C. Critical night lengths (CNLs) for ovipara (sexual female) induction were 6 h, 7 h and 8 h, respectively. High incidences of ovipara production were observed in all night lengths longer than the CNL including continuous darkness (DD), as well as in continuous light (LL) at 12.5 and 15°C. At the same three temperatures, the number of long- or short-night cycles required for half of the experimental aphids to be ovipara producers (i.e. the required day number, RDN) was determined. The RDN for long-night cycles (LD12:12) could not be determined at 12.5°C, but was temperature compensated between 15 and 17.5°C. The RDN for short-night cycles (LD20:4) could not be determined at any temperature. However, as induction of oviparae was always 100% in 12.5°C, 94–100% in 15°C and dropped from 100% to between 47 and 71% in 17.5°C, it seems that short-night accumulation was temperature dependent. When fourth-stadium larvae were transferred from LD20:4 at 20°C to the same light–dark cycle at 15°C, the aphids, when adult, switched to the production of oviparae after about 4 weeks. First-born progeny kept in LD20:4 and 15°C switched to the production of oviparae about 7 days after the moult to adult. Thus, the photoperiodic response can be directly affected by temperature, irrespective of photoperiod. Model-generated response curves using the ‘double circadian oscillator model’ for photoperiodic time measurement (Vaz Nunes, M., 1998. A double circadian oscillator model for quantitative photoperiodic time measurement in insects and mites. Journal of Theoretical 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 photoperiodic clocks (damping rate and period) as well as the photoperiodic counters.