Diurnal temperature amplitude is known to have a large influence on insect life history. Population density affects intraspecific competition and many other aspects of insect life history. However, there is limited information on the interactive effects of these factors on insects. Here, we tested the interactive effects of three diurnal temperature amplitudes (22 ± 0°C, 22 ± 6°C, and 22 ± 12°C) and three population densities on the development, survival, longevity, and fecundity of the English grain aphid Sitobion avenae (Fabricius) (Homoptera: Aphididae). At a constant temperature, increasing population density reduced the growth and survival of early-instar nymphs, increased longevity, and reduced fecundity. At a low population density, increasing temperature amplitude inhibited nymph development. However, even at a high temperature amplitude, nymph survival rate was higher than expected, and reproduction was possible because the recovery of the lower night-temperatures eliminated thermal stress. Increasing the population density reduced, and even reversed, the negative effects of the wide temperature amplitude. This may reflect synergistic interactions between population density and wide temperature amplitude as these stressors each incur energetic costs. These findings emphasize the importance of temperature amplitude and population density for improving prediction accuracy and damage assessment during pest control modeling.

Diurnal temperature fluctuations in nature can have a significant effect on many ectodermic traits. However, studies on the effects of diurnal temperature fluctuations on organisms, especially the effects on specific life stages, are still limited. We examined the immediate effects of the same average temperature (25°C) and different temperature amplitudes (±4, ±6, ±8, ±10, ±12°C) on the development and survival of Plutella xylostella (Lepidoptera: Plutellidae). We also assessed carry-over effects on adult longevity, reproduction, development, and survival of offspring across generations. The effect of moderate temperature amplitudes was similar to that of constant temperature. Wide temperature amplitudes inhibited the development of pupae, reduced total reproduction, lowered intrinsic rates of population growth, and slowed the development and survival of eggs on the first day, but the proportion of females ovipositing on the first three days increased. Insects coped with the adverse effects of wide temperature amplitudes by laying eggs as soon as possible. Our results confirmed that a logistic model based on daily average temperature cannot predict development rates under wide temperature amplitudes. These findings highlight the effect of environmental temperature fluctuations at the pupal stage on the development and oviposition patterns of P. xylostella and should be fully considered when predicting field occurrence.