Effect of Leaf Age on Length of Residual Activity of Abamectin in Pome Fruit Foliage

Abstract

The hypothesis that leaf age at time of application influenced absorption of abamectin and hence length of residual control was tested using field-aged residues in laboratory bioassays with twospotted spider mite, Tetranychu. S urticae Koch, on apple and pear foliage. Comparison was made with fenbutatin-oxide residues on apple foliage. Applications were made to 2-, 6-, or 12-wk-old leaves. On apple, initial mortality and the length of residual control by abamectin was highest for applications to younger leaves. The rate of decline in residual activity against T. urticae was steepest in the oldest leaves. With fenbutatin-oxide, leaf age did not affect initial activity, although the rate of decline in activity was higher when applied to older leaves. However, applications to older leaves coincided with weather conditions most likely to break down surface deposits of this material. On pear, initial residual activity for abamectin was lower in older leaves, but the rate of decline in activity was similar for all leaf ages. Overall, initial mortality and length of residual control were higher on pear than on apple, but best performance on both crops was obtained by applications to younger leaves. Implications for use of abamectin in apple and pear pest management programs are discussed.