Population Dynamics of the Walnut Aphid, Chromaphis Juglandicola (KALT.) in Northern California

Populations of waln ut aphids and associated insects were observed over a four year period in several northern California walnut orchards. Temperature, leaflet age, amount of prior aphid feeding and coccinellid predation were found to be the most important factors influencing walnut aphid population changes. Sharp declines in aphid population levels were correlated with high temperatures, especially when several days occur with maxima over 100°F. Temperatures may also affect aphids indirectly by directly affecting coccinellid beetles. Early spring temperatures influence the leafing out of the walnut trees. Cool spring temperatures can delay the leafing out of trees, thus postponing the aphid population increase. Carrying capacity of a leaflet for aphids is defined as a maximum number of aphids that can exist on a leaflet at any one time. This level appears to be determined by factors, other than space. Orchards in different areas and of various varieties, irrigation practices, and general vigor exhibit different carrying capacities for aphids. Factors which influence these levels are age of the leaflet and the amount of prior aphid feeding which has occurred on them. Coccinellid beetles were the most important predators encountered in the study. Examples are discussed where the following conditions prevailed: (1) where coccinellids were not involved, (2) where coccinellid population numbers trailed the aphid number and contributed only by shortening the time during which aphids were at maximal population levels, (3) where coccinellids lengthened the time required for the aphids to reach carrying capacity levels, and in so doing reduced the actual carrying capacities to some degree, (4) where coccinellids were the key factors regulating aphid numbers. Aphid parasitism was not important.