Problems in evaluating control of San Jose scale with spray chemicals

Problems of evaluating chemical control of San Jose scale are discussed on the basis of experiments in deciduous fruit orchards. Methods are described for sampling and counting scales for mortality determinations and for measuring the resurgence of a population after it has been reduced.

The methods used did not show conclusively any differences in performance among a number of the more effective insecticides used with oil in dormant or spring sprays. The methods were adequate to show that scale populations on peach trees do not increase as much in the growing season when trees are sprayed in May as when they are sprayed during fall or winter. On lightly infested peach trees sprayed in the dormant season or in May, the survivor progenies increased faster during the growing season than they did on trees that were more heavily infested when sprayed. The effects of variation in initial pest density on control were minimized by using pretreatment counts with the annual twig-count method. This method measured degree of population resurgence after treatment but did not show how numbers of survivors, effects of natural enemies, and other environmental factors acted together to produce the results observed. Sequences of samples for mortality determinations indicated trends in population change after treatment.