A Five-year Study of the Effects of the Walnut Aphid and the European Red Mite on Persian Walnut Productivity in Coastal Orchards12

Abstract

A five-year study of the effects of the walnut aphid, Chromaphis juglandicola (Kaltenbach), and the European red mite, Panotlychus ulmi (Koch), was conducted in a coastal Persian walnut, juglans regia L., orchard in California. By using four 4×4 Latin squares of single tree plots, selective pesticide regimes controlled or permitted aphid or mite populations that developed annually for 3 years, after which both species were controlled on all trees for 2 years. Yield reductions by aphids averaged 25% over the 1st 3 years, and continued at 28 and 43% during the 4th and 5th years after control was reestablished. Mite infestations had no effect on yield until the 3rd year when there was a 40% loss; losses of 22 and 31% persisted after control was reestablished. The symptom of effects upon the tree which proved to be the most sensitive to infestation by aphids or mites proved to be loss of productivity of staminate flowers (catkins). The effect on staminate flowers would be the most likely to reveal a damage threshold of such populations. After the 2nd and 3rd year of infestation, there was an average reduction in number of staminate flowers (catkins) of 68% attributable to aphids, 61% to mites, and 85% when both species were present. Aphid infestation reduced walnut size and increased shriveling of kernels, perforation of shells, and adhering hulls. European red mite had no effect upon nut quality. This experiment demonstrated that severe long-term effects by leafcell sucking arthropods on orchard tree productivity extend at least 2 years and probably much longer after return to control of infestation through chemical intervention.