Life History and Population Consequences of Density, Photo-Period, and Temperature in a Migrant Insect, the Milkweed Bug Oncopeltus

Density, photoperiod, and temperature exert profound effects on the life history and population growth of Oncopeltus. High temperature, long photoperiod, and low density result in earlier onset of reproduction, negatively skewed egg curves, and hence greater rates of population increase. Values or r (intrinsic rate of natural increase) vary from .0861 on a 16-hr day at 27⚬C with density of 10 pairs per container to .0369 on a 12-hr day at 23⚬C and 20 pairs. When r is low, length of life increases in partial compensation for suppressed reproduction. Photoperiod and density exert their influence only after adult eclosion, while temperature acts directly at all stages of life history to regulate the rate of development. The population responses of the phenotype to the environmental parameters studied are positive adaptations for a migrant insect such as Oncopeltus. It enters an essentially empty universe in the spring, when day length is increasing and temperatures are rising. The fact that all these factors lead to earlier reproduction with resultant higher population-growth potential means that Oncopeltus is well adapted for a rapid increase in numbers and full utilization of the habitat. Delay of reproduction by short days, low temperatures, and high densities may well represent incipient diapause; it is adaptive in that it allows the species sufficient time to migrate from an unsuitable area to complete its life cycle elsewhere.