Outbreaks of Two-Spotted Spider-Mites and Cotton Aphids Following Pesticide Treatment. I. Pest Stimulation vs. Natural Enemy Destruction as the Cause of Outbreaks1

Abstract

Numerous pesticides are known to provoke outbreaks of a variety of mites and aphids. Some of the conditions surrounding-these upsets are reviewed, and the capabilities of 59 different pesticides to induce increases of mites and/or aphids are examined in relation to their individual effects on some of the major natural enemies of mites and aphids. When critical examination is made of the arguments for either of the 2 suspected causes of such pesticide-induced outbreaks, i.e., (a) natural enemy destruction and (b) pest fecundity stimulation, it appears that neither by itself offers a completely adequate explanation. Since one of the methods now commonly used for evaluating natural enemy effectiveness relies on measurement of host increases in insecticidal check plots, this technique may credit the natural enemies with undeserved efficiency if the insecticide stimulates the pest’s fecundity. In this first of a series of experimental programs designed to resolve the problem of whether abnormal mite or aphid increases can be generated by pesticidal stimulation of the fecundity of mites or aphids, 59 different materials were examined for their effect on the reproductivity of the two-spotted spider mite, Tetranychus urticae Koch, and the cotton aphid, Aphis gossypii Glover. Following treatment with the different materials, population fold increases of each pest were measured weekly on the potted cotton plants held in the greenhouse and outdoors in the complete absence of natural enemies. The period of retention of toxicity of each material to mites and aphids (or the period during which any stimulation effect might be masked) is presented. Some anomalous conditions of treatment which caused abnormal increases in these tests could be explained by elimination of either pest when competing with the other. Some other abnormal increases of both mites and aphids were found which were difficult to explain other than as a pest-stimulation effect. For these cases no common denominator could be found. A few of the stimulatory responses obtained appeared in-consistent with the recorded performance of the materials in inducing upsets in the field.