Relative Abundance of Three Species of Spider Mites (Acari: Tetranychidae) on Cotton, as Influenced by Pesticides and Time of Establishment

Abstract

Cotton, Gossypium hirsutum L., was planted in experimental plots in the west central San Joaquin Valley of California, during two consecutive seasons. Plants were treated with dicofol, methyl parathion, and/or permethrin, to manipulate densities of spider mites, Tetranychus spp., within the plots. Spider mite densities were also modified by inoculative releases of Tetranychus turkestani Ugarov & Nikolski. Several times throughout each season, the relative abundance (i.e., proportion) of each of three spider mite species, Tetranychus urticae Koch, T. pacificus McGregor, and T. turkestani , was determined. In 1982, T. pacificus was the dominant species in all treatments, with a relative abundance of 0.85 in the dicofol plots, and 0.67 in the untreated control. However, T. pacificus was less dominant, and significantly lower in relative abundance in plots treated with methyl parathion (0.53) than in dicofol-treated plots. Conversely, T. turkestani was higher with methyl parathion (0.34) than with dicofol (0.13). In 1983, trends were similar to 1982, although T. urticae was the dominant species in the untreated control, with a relative frequency of 0.49. Permethrin and dicofol had a negative impact on the relative abundance of both T. turkestani and T. urticae . Populations of T. urticae exhibited susceptibility to dicofol, relative to the dicofol resistance of T. pacificus . Early-season inoculative releases of T. turkestani increased the relative abundance of T. turkestani especially in untreated plots. However, late releases were less effective than early releases in promoting the buildup of T. turkestani , relative to the other two species.