CONTAGIOUS DISTRIBUTION IN THE EUROPEAN RED MITE, PANONYCHUS ULMI (KOCH), AND A METHOD OF GRADING POPULATION DENSITIES FROM A COUNT OF MITE-FREE LEAVES

Examination of a large number of 100-leaf samples shows that the European red mite is distributed contagiously, not randomly, on leaves of apple trees. When the mean number of mites per leaf is plotted against the proportion of mite-free leaves for the corresponding sample, the points fall in a narrow zone, steepest and narrowest at the lower densities. Following application of miticides to heavily infested trees, it is usual to summarize the findings of the tests by grading the performance of the chemicals as excellent, good, etc. according to mite density as determined by s total count from the 100-leaf sample. A method is outlined, in which, by making use of plotted values, a count of mite-free leaves only is sufficient to allow the assessment of performance in such categories.