Density, Spatial Distribution and Sequential Sampling Plans for Cereal Aphids Infesting Wheat Spike in Gorgan, Northern Iran

The seasonal abundance patterns of the wheat spike aphids were studied by weekly sampling of five winter wheat fields in Gorgan region in northern Iran during two growing seasons، 2006 and 2007. Spatial distribution of different developmental stages and morphs of the aphids were described by fitting data to Poisson (random) and negative binomial (clumped) distributions، as well as by calculating dispersion indices. A sequential sampling plan was also developed using the fixed-precision method of Green for estimating the density of adults، nymphs and total population. Sitobion avenae (F.) and Schizaphis graminum (Rondani) were two main species infesting wheat spikes in this region. The first aphid colonies appeared on spikes during early April and peaked (16. 01±2. 86 aphid per spike) during early May. Based on R2 and F of regression analysis، Taylor’s power law provided a more adequate description of variance/mean relationships than did Iwao’s patchiness regression model، and the b values ranged from 1. 034 (alate adults) to 1. 545 (apterus adults). Aphid population، especially nymphs and apterous females، was aggregated during most of the growing season and negative binomial models fitted data sets better than the Poisson series. However، alate morphs showed a noticeable tendency to the random distribution and 63. 7 percent of their data sets fitted the Poisson distribution. Regarding 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 0. 25. Optimum sample size was flexible and depended upon the aphid density and desired level of precision، and ranged from 25 spikes in density of 16. 01 aphids/spike to 268 spikes in 0. 168 aphids/spike at the precision level of 0. 25. The information presented in this study may be used in population management of aphids in wheat fields.