The control of Pieris rapae with DDT. III. Some changes in crop fauna.
The following are virtually the author's summaries of these parts of a series on observations in central England [cf. RAE A 57 7969].
Some of the changes in the fauna of a crop of brussels sprouts following control of P. rapae with DDT are described. Species differed in the extent to which they came into contact with the insecticide, their susceptibility to it and their powers of recolonisation, and the direct effects of DDT on the numbers of any two species were therefore sometimes very different. Furthermore, component species of the crop fauna were interdependent and a change in the numbers of one inevitably had repercussions on others. In consequence, the application of a non-specific and persistent insecticide such as DDT had effects that were complex and often unpredictable. Some animal groups such as millepedes, centipedes, Dipterous and Coleopterous larvae and Mesostigmatid mites were particularly sensitive to it, and their powers of recolonisation were poor. Others, such as Carabids and Staphylinids, were far less sensitive and were active enough to recolonise the crop each winter after cultivation. Plant-living species were generally less affected than ground-living ones, as the effective persistence of DDT was less on the plants. The commonest cause of an indirect effect of the insecticide on a species was the reduction or elimination of one of its natural enemies [cf. above]. This led to an upsurge of a number of species after spraying, including P. rapae and Brevicoryne brasstcae (L.). Another type of indirect effect occurred with two Carabids, Trechus quadristriatus (Schr.) and Nebria brevicollis (F.). These increased in number following an increase in the number of Collembola, which formed their principal prey. The basic cause of all of the effects described was the non-specificity of DDT, but its persistence greatly added to its effect by preventing recolonisation by many species.