Effect of photoperiod on the duration of summer and winter diapause was investigated in the cabbage butterfly, Pieris melete. By keeping naturally induced aestivating and hibernating pupae under various photoperiods, it was shown that diapause duration of aestivating pupae was significantly longer at long than at short daylengths, whereas diapause duration of hibernating pupae was significantly shorter at long than at short daylengths, suggesting both aestivating and hibernating pupae require opposite photoperiodic signals to promote diapause development. By transferring diapausing pupae, induced under various photoperiods, to 20°C with a naturally changing summer daylength, the diapause induced by short daylengths was easier to terminate than diapause induced by long daylengths. When naturally induced aestivating and hibernating pupae were kept under natural conditions, aestivating pupae had a long diapause (mean 155 days) and wide range of emergence (90 days), whereas hibernating pupae had a short diapause (mean 105 days) and a relatively synchronized emergence (lasted 30 days). Finally, the ecological significance of photoperiodic regulation of diapause duration is discussed.
Preferences of young caterpillars of three species of Pieris (P. rapae crucivora Boisduval, P. melete Ménétriès, and P. napi japonica Shirôzu) (Lepidoptera: Pieridae) for the upper and lower surfaces of the leaves of their host plants (Brassicaceae) were investigated in the laboratory. On horseradish Armoracia rusticana Gaertn. Mey. et Scherb., which was provided as a common food for three species, second and third instar larvae of the respective species preferred the lower to the upper surface of horizontally placed leaves, irrespective of whether they hatched on the upper or lower surface. First instar larvae seemed to remain on the surface on which they hatched. However, first instar larvae of P. melete on Rorippa indica (L.), a natural food of P. melete in the field, and first instar larvae of P. napi japonica on Arabis flagellosa Miq., a natural food of P. napi japonica, preferred the lower to the upper surface, just as second and third instar larvae did. To elucidate the effects of leaf-surface preference, the percentage parasitism of P. rapae crucivora on Arm. rusticana and Ara. flagellosa by the parasitoid Cotesia glomerata (L.) (Hymenoptera: Braconidae) was investigated. On Arm. rusticana, the percentage parasitism of the larvae on the upper surface was higher than that of larvae on the lower surface. On Ara. flagellosa, however, percentages parasitism were nearly equal on both surfaces. Leaf-surface preference by the larvae of Pieris is discussed in terms of avoidance of parasitism by the parasitoid C. glomerata.
Effect of pre-diapause temperature on summer and winter diapause intensity was examined under both laboratory and field conditions. Under short photoperiods of 8L : 16D and 10L : 14D, all pupae entered diapause at 15, 18 and 20°C and the incidence of diapause dropped to 82.3% and 85.5% at 22°C, respectively. Under long photoperiods of 14L : 10D and 16L : 8D, the incidence of diapause decreased with increasing temperature and there were significant differences among temperatures. The incidence of diapause at 16L : 8D was significantly lower than that under14L : 10D at 20 and 22°C. By transferring diapause pupae induced under various temperatures (18, 20 and 22°C) at a short day of 10L : 14D or a long day of 14L : 10D, to 12.5L : 11.5D, 20°C, the duration of summer diapause induced under 22°C (mean 76.1 days) was significantly shorter than those under 20°C (mean 85.9 days) and 18°C (mean 90.9 days), showing that the incidence of summer diapause was positively linked to the intensity of summer diapause; whereas the duration of winter diapause induced under 10L : 14D was similar at 18°C (89.2 days), 20°C (88.7 days) and 22°C (89.2 days) and there were no significant differences. Field experiments also showed that the high rearing temperatures significantly decreased the incidence and intensity of summer diapause, but had no significant affect on the intensity of winter diapause. When the naturally aestivating pupae from the first spring generation (formed on 24 April) and second spring generation (formed on 15 May) were kept under summer conditions, the diapause duration of the first generation lasted for 107-166 days (mean 146 days), about twenty days longer than that of the second generation [lasted for 92-151 days (mean 126 days)]. All results reveal that the sensitivity to temperature prior to aestivation and hibernation was quite different.