Temperature-dependent life tables were constructed of an insect cohort (i.e., of a group of individual insects of the same species and age). Observations started from “eggs,” all laid within the last 12–24 hr. Owing to natural mortality within each of the immature life stages (egg, larva, and pupa), a minimum of 100 eggs were used (n ³ 100). Depending on the insect species, complete life-table or single-stage cohort studies were employed. Life-table data were collected over a range of constant temperatures (at 10°, 15°, 20°, 25°, 30°, and 35°C) in which a species could develop. This was done to understand each species’ minimum and maximum thresholds for development.
In complete life-table experiments, a group of individuals of the same age were observed from the beginning of their egg stage until the death of all adults. During insect development, all phenological events (i.e., development time of immature life stages, mortality, longevity, and fecundity) were recorded at constant intervals of generally 1 day. In single-stage cohort studies, individuals of the same age of each developmental stage (egg, larva, pupa, and adult) were used. The phenological events were monitored until the development to the next stage or death of all individuals in the cohort.
Data on the daily oviposition at each constant temperature were collected, which included the variation in the female rate and the proportion of females in the progeny. In complete life tables, at least 30 females were included in the assessment of fecundity. The number of insects used for constructing a life table at extreme high and low temperatures, where mortality is generally high, was increased because of the expected increase in mortality during immature life stages. The number of individuals used was not necessary balanced. Analysis of the data included weights that account for differences in numbers of individuals entering a certain life stage at a given temperature. In some cases, life tables were repeated at the same temperatures with another batch of individuals from the population. During the analysis, data from the same temperature (replications) was pooled when fitting models for describing temperature effects on insect development or fecundity.