Population Density and Spatial Distribution of Aphis gossypii Glover (Homoptera: Aphididae) on Cotton in Gorgan, Iran

The seasonal abundance patterns of the cotton aphid, Aphis gossypii Glover, in cotton fields at Gorgan in northern Iran were studied during two growing seasons of 2002 and 2003. The spatial distribution of different developmental stages and morphs of the aphid was described by fitting data to Poisson (random) and negative binomial (aggregated) distributions, and calculating the dispersion indices. A sequential sampling plan was also developed using the fixed-precision method of Green for estimating the density of the adult, nymph and total population. The first aphid colonies appeared on plants during late June and early July and peaked in early September when cotton plants were at the boll maturation and opening stages. Aphid populations, especially nymphs and apterous females, were aggregated during most of the growing season and negative binomial models fit data sets better than the Poisson series. The percentage fit for alate morphs showed a slight tendency to the Poisson distribution. With respect to sampling cost or required sample size, the developed fixed-precision sequential sampling plans showed an acceptable performance for estimating aphid density at the precision level of D= 0.25. The optimum sample size was flexible and depended upon the aphid density and desired level of precision, and generally ranged from 10 to 513 and 62 to 3,206 at the precision levels of 0.25 and 0.10, respectively. The sequential sampling plans developed could be recommended to estimate the aphid density in integrated pest management programs.