Plant-quality effect on life-history parameters of the twospotted spider mite (Acari: Tetranychidae) on cotton

The suitability of cotton for the development and reproduction of the two-spoked spider mite, Tetranychus urticae Koch, was assessed. Mites were reared on young and old cotton leaves at several times of the season to determine if plant quality influenced the early-season decline in mite populations on cotton or their within-plant distribution. The developmental time, preoviposition period, reproductive period, and life span of mites was influenced only slightly by leaf age or stage of plant development. However, lifetime fecundity was lowest on cotyledons and on old leaves. Fecundity on young leaves was high for most of the season but declined when plants reached boll maturation. The low suitability of cotton cotyledons may slow the development of mite populations early in the season but only until true leaves develop. Nitrogen content of leaves declined as plants developed and was lowest in older leaves. In contrast, tannin content increased progressively with plant development and was higher in younger leaves than in older leaves. Mite developmental time was related negatively to nitrogen content, whereas fecundity was related positively. In preference tests, mites strongly preferred to feed and oviposit on younger leaf tissue compared with older leaf tissue. In the field, mites released near the tops of plants remained there, whereas those released onto leaves in the middle and bottom of plants moved upward, either along the mainstem or along branches, to younger leaves. Rearing studies indicate a fitness advantage will accrue to mites developing on younger leaves, possibly because of the higher nitrogen content than in older leaves. The within-plant distribution of mites is a result of both the ability of mites to assess leaf quality and a likely positive phototaxis, which directs the mites toward younger leaves.