Effects of Survival Rate and Fecundity on the Intrinsic Rate of Increase of Helicoverpa Armigera (Hübner) (Lepidoptera: Noctuidae)

Abstract

The life history data of Helicoverpa armigera (Hubner) (Lepidoptera: Noctuidae) kept at 25°C on an artificial diet, on hybrid sweet corn, and on asparagus, as well as those kept at 29°C on an artificial diet were analyzed to assess the effects of survival rate and fecundity on the intrinsic rate of increase (r). The curtailed intrinsic rate (rδ) describes the effects of survivorship and fecundity on the intrinsic rate of increase. The curtailed intrinsic rate of H. armigera was estimated by applying the age-stage, two-sex life table theory, and ranged from negative to positive and finally became equal to the intrinsic rate. The cumulative contribution (ry) is a descriptive value of the cumulative input of individuals surviving to age y to the intrinsic rate. The value of cumulative contribution (ry) ranges from zero to r, however, it is not an intrinsic rate. At the beginning of a life table study of H. armigera using a fixed hatch rate, a cohort consists of unfertile eggs, and calculating the fecundity from the total laid-eggs by ignoring their hatchability does not reflect the true biological characteristics of H. armigera. Rather, it shifts the survivorship curve downward, the fecundity curve upward, and provides very high estimates of the life table parameters. In addition, it reduces the reliability of the estimate of the intrinsic rate of increase to be used as an indicator to evaluate the performance and fitness of H. armigera under variable conditions. Therefore, we recommend that the fecundity be calculated using the number of viable eggs. In addition, the age-stage, two-sex life table should be used in order to accurately analyze the data to encompass the age-specific variation in sex ratio, hatchability, and development in the analysis.