UC Berkeley

An understanding of how fertilizer input affects pest populations and, consequently, the need for pesticide use would allow for the development of more effective integrated management strategies. The present study addressed the response of walnut aphid Chromaphis juglandicola (Kaltenbach) (Hemiptera: Aphididae) populations to different levels of nitrogen application to walnut seedlings and the relationship between aphid density and chlorophyll content index (CCI), a non-invasive measure of foliar nitrogen, for mature trees in commercial walnut orchards. Although added nitrogen significantly increased soluble nitrogen content, CCI and soluble nitrogen:phosphorus ratios in the foliage of walnut seedlings, there was no effect on walnut aphid population growth. Similarly, there was no relationship between aphid density and CCI for mature trees in the field. A neutral response suggests that walnut aphid population growth on potted seedlings is limited by factors other than soluble nitrogen, such as other nutrients, amino acid composition or defensive compounds. Consequently, fertilizer management appears unlikely to effectively contribute to the control of C. juglandicola in commercial walnut orchards.