Presence-Absence Sampling of Twospotted Spider Mite (Acari: Tetranychidae) in Pear Orchards

A sampling plan was developed for rapidly assessing densities of Tetranychus Urticae Koch in pear ( Pyrus communis L.) orchards. To estimate the mean number of mites per leaf on a 4-ha block of pears, a sample of leaves taken from the inner-base area of each of 10 randomly chosen trees was checked for the presence-absence of twospotted spider mite on each leaf. The mean number of mites per leaf ( Y ) can be estimated from the proportion of leaves infested ( X ) according to the equation log (Y + 1) = −0.0015 + 1.0859X. The coefficient of determination for this equation, derived from 219 records of X and Y, was 0.94. Conservative-action thresholds to prevent premature leaf drop were derived as 2.1 mites per leaf (32% of leaves infested) and 1.2 mites per leaf (22% of leaves infested) respectively for the pear varieties ‘Packham’s Triumph’ and ‘Williams’ Bon Chretien’.