Population fluctuations of small walnut aphid Chromaphis juglandicola (Hemiptera: Aphididae) and its natural enemies in walnut orchards of northwestern Iran

Small walnut aphid (SWA) Chromaphis juglandicola is an economic pest of walnut trees. Population fluctuation of SWA was studied in Mamaghan walnut orchards (East Azarbaijan Province, Iran).Three factors affecting population density of SWA, including vertical divisions of canopy (upper and lower half), geographical orientations of canopy (at four levels) and elongation of branch (basal and distal ends), were investigated. Sampling unit was a cluster of five leaves. Totally ۱۲۸ samples were taken weekly from all strata upon eight chosen trees, repeating ۳۰ times during the season. Natural enemies also were counted. The first SWA individuals were observed early May, consisting of first instar larvae and alate females. A sudden population increase occurred in mid-May. Maximum ۴۰ aphids/leaf were observed in early June with first and second instars dominant, followed by a sudden decline in late June. A small peak was observed at early October. A partial tendency was observed toward north of canopy, at basal half of downward branches. Positive linear correlation between natural enemies' and SWA populations suggests density dependence. Moreover ۲-۴ week delay was present between them. Trioxys pallidus (Holliday) was dominant natural enemy in the region. Contrary to previous works, overwintering stages of the SWA in the region were predominantly developed stages (third and fourth instars as well as pre-reproductive winged females).