Data on pupation and emergence dates for the nymphalid Purple Emperor butterfly Apatura iris have been collected at Basel, Switzerland, between 1982 and 2002. The butterfly has been shown to emerge on average 9 (males) to 12 (females) days earlier per decade, 19 and 24 days earlier respectively over the study period. Emergence dates relate strongly to spring temperatures, particularly with daily maximum temperatures for the months March to May. Temperatures for these months have increased significantly during this period (0.7°C to 1.8°C per decade). Three factors suggest that the strongest influence of the rise in spring temperatures has been on late larval instar growth and development: (i) May temperatures dominate emergence date models and larvae are feeding faster and for longer periods during this month, (ii) Salix caprea flowering date, a surrogate for bud burst, is excluded in stepwise regression models with temperatures and years suggesting that tree phenology may be less important than temperature effects on later development, and (iii) convergence of female and male emergence dates over time points to limits on earlier feeding in protandrous males. A negative consequence observed with earlier emergence dates is lethal extra broods.
Under seasonal conditions, Polydesmus angustus individuals born in the first part of the breeding season have a 1-year life cycle and those born later have a 2-year life cycle (cohort-splitting). In this study, 249 juveniles from four early broods (born in mid-July) and four late broods (born in September) were reared under similar laboratory conditions, to test for possible maternal influences on life-cycle duration. Development times of early- and late-born individuals were compared under four combinations of day length and temperature (16 h - 18°C, 16 h - 16°C, 12 h - 18°C and 12 h - 16°C). The results showed that development time varied significantly in response to day length, temperature and sex, but that of individuals in the early and late broods did not differ significantly (mean development times ± SE: 180 ± 6 and 183 ± 8 days, respectively). There were no significant interactions between birth period and other factors, indicating that the effects of day length, temperature and sex on development time were similar in early- and late-born individuals. This indicates that the extended life cycle of millipedes born late in the season is not maternally determined and that cohort-splitting is controlled entirely by the environmental conditions experienced by the offspring during their development. This conclusion is supported by the absence of significant variation in offspring live weight at birth measured at different times in the breeding season. The results are discussed in relation to the bet-hedging theory, which is often put forward to account for cohort-splitting in arthropods. In P. angustus, the results are consistent with either bet-hedging or adaptive plasticity, but further studies are required to decide which interpretation is correct. and Jean-François David, Jean-Jacques Geoffroy.
The influence of photoperiod on the thermal requirements for development was discovered for the first time in insects during experiments on the linden-bug, Pyrrhocoris apterus. The effect of photoperiod on the duration of linden-bug development at five constant temperatures (20, 22, 24, 26 and 28°C) was measured and the thermal requirements for development at three photoperiods (14, 17 and 20 h light per day) were calculated. Bugs from four geographic populations were used in these experiments: Pyatigorsk (44°02´N, 43°04´E), Borisovka (50°36´N, 36°01´E), Mikhailov (54°15´N, 39°0´E) and Ryazan (54°36´N, 39°42´E). From the values of individual development times at different temperatures the coefficient of linear regression of development rate (the inverse of the duration) on temperature and the thermal threshold for development were calculated. Both these parameters were found to decrease significantly with decrease in day-length for all four populations studied. It means that at shorter day-lengths nymphal development is less dependent on temperature compared to the development at longer day-lengths. These effects seem to be adaptive. The development times of nymphs at relatively high temperatures (above 24-25°C) are shorter under long-days than under short days which should be advantageous at the height of summer when the days are long and the weather is warm. In the contrast, at relatively low temperatures (below 24-25°C) the nymphs develop significantly faster under short-days than under long days, which is advantageous at the end of summer as it allows the nymphs to reach the adult stage, the only stage capable of overwintering. The influence of photoperiod on the thermal reaction norm appeared to be more or less gradual, i.e. the shorter the day-length the shallower the slope of the regression line of development rate on temperature and the lower the thermal threshold for development. An analysis of the literature shows that this effect of photoperiod on the thermal requirements for development is widespread among insects but has been overlooked by previous authors. The authors conclude that the variation in the development time observed in insects at different seasons, photoperiods or food regimes, or from different populations, etc., are generally due to some modification of the thermal reaction norms and more specifically to differences in the thermal requirements for development.
Life-history parameters (juvenile development time, adult longevity, host instar preference and rate of parasitism) of four parasitoids of Bemisia argentifolii (two strains of Encarsia formosa (D and B), Eretmocerus eremicus and Eretmocerus mundus) were studied in the laboratory. At 15°C juvenile development time was the shortest for E. formosa B (48 days), longest for E. eremicus (79.3 days) and intermediate for E. formosa D (62.8 days) and E. mundus (64 days) at 15°C. With increase in temperature, development time decreased to around 14 days for all species/strains at 32°C. The lower developmental threshold for development was 11.5, 8.1, 13.0 and 11.5°C for E. formosa D, E. formosa B, E. eremicus and E. mundus, respectively. E. formosa D and B, and E. mundus all appeared to prefer to parasitize 3rd instar nymphs. The presence of hosts shortened adult longevity in most of the parasitoids, with the exception of E. formosa B, which lived longer than other species/strains irrespective of the presence of hosts. At 15°C daily parasitism was very low by all parasitoids. The two Encarsia strains had a constant, but low rate of reproduction during adult life, while the two Eretmocerus species had a very high rate of reproduction when one-day old, which then decreased very quickly. Lifetime fecundity, estimated using a non-linear model, indicated that it was higher for the two Encarsia strains than for the Eretmocerus species. Life history parameters reported in the literature for the four parasitoids are reviewed and compared with our results. Finally, the potential value for the biological control of whiteflies on greenhouse crops of parasitoids having either a high reproductive rate over a short period (Eretmocerus spp.) or a low rate of reproduction over a long period (Encarsia spp.) is discussed.