A spider mite-resistant and a spider mite-susceptible variety of cotton were planted in the field in a replicated design. An acaricide (dicofol) and an insecticide (acephate) were used to manipulate numbers of spider mites and their natural enemies. In general, numbers of spider mites remained low until just before midseason when 650 °D air temperature had accumulated. At 650 °D, mite numbers began to increase more sharply and reached significantly (P<0.001) greater levels on susceptible plants than on resistant plants. Although natural enemies, such as Geocoris pallens Stål, Orius tristicolor (White), and Frankliniella occidentalis (Pergande), were slightly more apparent on susceptible plants than on resistant plants, these differences were not statistically significant, and cotton varietal resistance was compatible with biological control.

Acephate significantly (P<0.05) reduced predator densities for up to 2 weeks following each application, and after 550 °D mite populations had begun to explode on plants sprayed with acephate. Predators had a greater impact on mites on susceptible plants than on resistant plants, which was attributed to greater densities of spider mite prey on susceptible plants. After 900 °D, spider mites were out of control on susceptible plants treated with acephate, and the acaricide dicofol had no observable effect on mite numbers. Rapidly increasing mite populations appear to exhibit acaricide resistance, solely by attaining very high densities. Dicofol efficacy was relatively high on spider mite-resistant plants, because mite growth rates were slower, and thus mite populations could not “outgrow” the effect of the acaricide. Hence, there appeared to be a synergistic interaction between acaricide and varietal resistance.

In untreated plots, yield was significantly (P<0.05) higher in the susceptible variety than in the resistant variety. However, this trend was reversed in plots treated with acephate, and yield from resistant plants was significantly (P<0.05) greater than yield from susceptible plants. On crops heavily treated with insecticides, resistant varieties may offer the best alternative for pest management.