Spider Mites (Acari: Tetranychidae) Affect Yield and Fiber Quality of Cotton

The effect of twos potted spider mites, Tetranychus urticae Koch, on the yield and fiber quality of cotton was studied at high-yield levels in Australia over four field seasons. Significant reductions in yield, fiber quality, and seed viability were observed and were found to be related to the time of infestation and rate of development of twos potted spider mite populations. Early infestations (at early flowering), which increased quickly, caused the greatest reductions whereas later infestations (at first boll opening), which increased more slowly, had little effect on yield. Early twospotted spider mite infestations reduced the number of bolls set and boll size, but later infestations affected boll size only. It is proposed that by reducing the photosynthetic capacity of leaves, severe infestations of twos potted spider mite may heighten competition for assimilates among sinks within the plant (e.g., fruit and leaves), potentially causing fruit shed and limiting the development of bolls. Regression analysis indicated that yield reductions can be predicted from the rate of increase of twospotted spider mite infestations, the date at which they begin to increase, and the length of the growing season. Using this information it is possible to identify populations that are increasing at rates capable of reducing yield, allowing control to be implemented if necessary. The rate of increase of twospotted spider mites was lower on the okra leaf cultivar Siokra, than on the normal leaf cultivar, Deltapine 90, indicating that Siokra is more resistant. Identification of the mode of resistance may reveal ways in which resistance to twos potted spider mites can be further enhanced.