The control of Pieris rapae with DDT. I. The natural mortality of the young stages of Pieris.
The following is based largely on the authors summary. This paper is the first of a series describing observations on a population of Pieris rapae (L.) on brussels sprouts in central England and some of the other species inhabiting the crop. The natural mortality of Pieris was studied during 1964-66 to provide background information on the interrelations between different species and the pest. Life tables constructed for three generations of Pieris showed that there is about 90% mortality between the egg and pupal stages [cf. RAE A 55 2466]. Over half occurred during the first two larval instars and was primarily due to arthropod predators. Application of the precipitin test showed that 12 species had fed on Pieris. By far the most important were Harpalus rufipes (Deg.) and Phalangium opilio L. As the larvae grew, arthropod predators became less important. It was estimated that birds took about 20% of the larvae, but they tended to take the larger ones, so that mortality due to them was important only during the last two larval instars. The larvae were attacked by four insect parasites, Apanteles rubecula[Cotesia rubecula] Marshall, A. glomeratus (L.) and the Tachinids Phryxe vulgaris (Fall.) and Epicampocera succincta (Mg.), but none contributed significantly to the total generation mortality of Pieris although A. rubecula killed about 20% of the fourth-instar larvae in 1964. Two disease organisms were recorded; the fungus Metarrhizium anisopliae had an insignificant effect, while a granulosis virus, probably Bergoldiavirus virulentum killed 1-4% of the larvae. In all years, a few larvae drowned during heavy rain, but the main effect of weather on the Pieris population was exerted not through mortality but through fecundity. The number of eggs laid appeared to be closely dependent on weather and can be greatly reduced by cool, cloudy conditions.