Spatial Distribution of Chromaphis juglandicola (Kaltenbach) (Homp., Aphididae) in East AzerbaijanWalnut Orchards

Small walnut aphid (SWA), Chromaphis juglandicola (Kaltenb), is a pest of walnut trees in many regions. Population fluctuations as well as spatial pattern of distribution of small walnut aphid (SWA) was studied during growing season of 2012 by weekly samplings in Mamaghan walnut orchards, East Azerbaijan, Iran. Sampling program was conducted for about six months from late April to November. All stages of aphid were counted separately on 128 clusters of leaves collected from eight walnut trees. Discrepancy of the sampling data from whether Poisson (random) or negative binomial (aggregation) statistical distributions was investigated by Pearson's Chi square test. Disregarding stages of SWA, goodness of fit tests were carried out with total populations. Contagiousness indices as well as Taylor's power low were also used to distinguish the pattern of distribution. In all samplings from 10 May to 10 August, data fitted to negative binomial distribution only with two exceptions. Afterward population deeply declined and well fitted to random pattern up to 20 September. Relative increase of population in late season leads to a tendency to aggregation once again. As a conclusion, SWA colonization begins as random; aggregation occurs following on site reproduction of females and finally random situation returns following depletion of patches. A second aggregation occurs late in the season when autumnal natality regains. Taylor's power low also showed a clumped pattern with a slope of b=1.55.