Field Measurement of the Dependence of Life History on Plant Nitrogen and Temperature for a Herbivorous Moth

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Abstract
(1) A generation of Samea multiplicalis Guenee (Lepidoptera: Pyralidae) was reared in field cages in each season of a year on plants with either high or low nitrogen content (N%). Development rate and survival of all stages, and adult fecundity were related to temperatures and plant N% using multiple regression. (2) Development rates of all stages were greatest at 28-30 .degree.C, declining significantly at lower and sometimes at higher leaf temperatures. Early larval survival declined sharply at temperatures over 30 .degree.C, but was largely unaffected by low temperatures. Pupal survival declined with decreasing leaf temperatures. Fecundity was greatest at air temperatures of 20-22 .degree.C, and egg survival at leaf temperatures of 25-26 .degree.C. (3) Declining plant N% had pervasive effects throughout the life history; reducing larval growth rate and survival, slowing pupal development, reducing fecundity and slowing development of eggs laid, but increasing the survival of the eggs. The effects of temperature on the life history were modified by plant N%. Larval development rates were greatest at lower temperatures on lower plant N%, and the heavy early larval mortality at high temperatures was exacerbated by lower plant N%. Finally, differences in intrinsic rate of population increase between N% treatments were greatest in higher temperatures, and least in winter temperatures. (4) Results supported the hypthesis that the larval period was adaptively plastic rather than simply physiologically constrained. The larval development period and response of larval period to plant N% differences were significantly heritable. The coefficient of variation in larval period was twice that of non-larval periods combined. Variation in generation time could be explained almost entirely by variation in larval period. (5) The differential survival of ''hardier'' genotypes when early larval mortality was high, may explain some of the variation in life history of later stages. In particular, the higher survival of eggs of females on low plant N% may represent a difference in genotypes following heavy early mortality. (6) Intrinsic rate of population increase was highest in autumn for both low and high N% treatments, and least in summer for the N% treatment, as observed for natural populations. This evidence suggests that plant N% and temperature were the most important factors in the population growth of S. multiplicalis.