The control of Pieris rapae with DDT. II. Survival of the young stages of Pieris after spraying.
The following are virtually the author's summaries of these parts of a series on observations in central England [cf. RAE A 57 7969].
Following the use of DDT to control the larvae of Pieris rapae (L.) on brussels sprouts, survival of the pest was markedly improved because the insecticide killed many of its natural enemies. Most of the difference between survival on sprayed and unsprayed crops was due to the action of DDT on arthropod predators. By far the most important of these were ground-living species that climb the plants at night and feed on the young larvae, such as the Carabid Harpalus rufipes (Deg.), and the Arachnid Phalangium opilio L. These were reduced in number by the presence of DDT both in the soil and on the foliage. Predators living on the plants, such as spiders and Anthocorids, were also reduced. Syrphid larvae were resistant to the insecticide, but their number was also reduced because fewer Pierìs eggs were laid on the sprayed crop. Of the insect parasites, Apanteles rubecula[Cotesia rubecula] Marshall was eliminated from the crop each year by spraying, but others were less affected. Parasites were of only minor importance in comparison with predators. In all years the incidence of the granulosis virus disease [cf. loc. cit.] was higher on the sprayed than on the unsprayed crop, indicating that sublethal doses of DDT may increase the susceptibility of Pieris to the disease. A foliage application in the current year had the biggest effect on natural enemies, since it affected both ground-and plant-living species. The ground-living species were, however, so important that residues in the soil from applications in previous years markedly reduced the mortality they caused.