The Control of Pieris rapae with DDT. I. The Natural Mortality of the Young Stages of Pieris

Abstract

The natural mortality of Pieris was studied to provide background information on the interrelationships of different species and the pest. Life tables were constructed for 3 generations of Pieris. These show that there is a mortality of about 90% between egg and pupal stages. Over half of this mortality occurred during the 1st 2 larval instars and was primarily due to arthropod predators. Twelve species were shown, by means of the precipitin test, to have fed on Pieris. By far the most important were the ground beetle, Harpalus rufipes, and the harvestman, Phalangium opilio. As the caterpillars grew, so arthropod predation became less important. It was estimated that birds took about 20% of the larvae. Birds tended to take the larger caterpillars, so mortality due to them was only important during the last 2 larval stages. Four species of insect parasite attacked the caterpillars of Pieris. These were 2 species of Apanteles (A. ru-becula and A. glomeratus) and 2 species of tachinid fly (Phryxe vulgaris and Epicampocera succinata). None of these contributed significantly to the total generation mortality of Pieris, although A. rubecula killed about 20% of the 4th stage larvae in 1964. Two disease organisms were recorded. Of these a fungus (Metarrhizium anisophliae) had an insignificant effect, while a granulosis virus (probably Bergoldia virulenta) killed from 1% to 4% of the caterpillars. In all years a few larvae drowned during heavy rain, but the main effect of weather on the Pieris population was not on mortality but on fecundity. The number of eggs laid appears to be closely dependent on weather, and can be greatly reduced by cool, cloudy conditions.