Effects of Spider Mites on Almond Tree Growth and Productivity1

Abstract

Two well replicated field experiments on the effects of mite infestation were conducted in almond orchards in the southern San Joaquin Valley of California. The orchards were previously protected with acaricides from infestation by Tetranyshus spp. and citrus red mite, Panonychus citri (McGregor). The severe mite infestations which develop on almond in this area are not related to use of insecticides, replicated comparisons were made in both experiments by comparing growth and productivity of protected and unprotected trees. High mite populations that developed in summer did not adversely affect almond tree growth and productivity during the year in which the infestations occured. However, in the year following infestation, yields of unprotected trees were an ave 13% lower ( P =0.05) in an orchard then in its 6th yr and 19% less ( P =0.05) in an orchard then in its 4th yr, as compared with yields of trees protected with an acaricide. Similarly, in the year following infestation, length of terminals on unprotected trees was reduced 11% in the orchard in its 6th yr (not significant) and 48% ( P =0.01) in the orchard in its 4th yr. Trunk growth of unprotected trees was reduced an avg 19% ( P =0.05) in the year following mite attack, the 6th season of growth in the orchard.