A study of factors regulating populations of Pieris rapae Linnaeus in Canterbury, New Zealand

The small white butterfly, Pieris rapae Linnaeus, was accidentally introduced to New Zealand in 1930. In the absence of its specific natural, enemies, P. rapae multiplied rapidly and-by 1936 posed a serious economic problem to growers of brassica crops. The importation of insect parasites of the larval and pupal stages resulted in a significant decrease in the population level of P. rapae. A granulosis virus, possibly imported accidentally with one of the parasites, has also been cited (Kelsey, 1962) as a major factor in the reduction of populations of P. rapae in New Zealand. Today, P. rapae is rarely a problem in agricultural crops and application of insecticides is generally unnecessary. In horticultural crops, however, where the acceptable level of damage is much lower, insecticides are used against P. rapae at regular intervals throughout the season. Population studies of P. rapae, in New Zealand, have not been carried out previously, and quantitative data on any of the factors affecting P. rapae are meagre. The aim of the present study was to examine, as fully as possible, the factors regulating populations of P. rapae in Canterbury and thus to determine why populations of P. rapae maintain a relatively steady level from season to season. A further aim of the study was to investigate the effect of insecticide applications on the natural mortality factors influencing P. rapae. The experimental part of the study was concerned therefore with determining the most suitable method for sampling field populations of P. rapae, in order to provide reliable estimates of mortality. It was then necessary to develop techniques which could be integrated with the sampling method, to permit a partitioning of the measured mortality between the various mortality factors.