تغییرات تراکم، پراکنش فضایی و نمونه‏برداری دنباله‏ای جمعیت شکارگرهای مهم شته‏خوار در مزارع گندم منطقه‎ی گرگان

Abstract
In order to investigate density fluctuation and spatial distribution of the main aphidophagous predators and develop a fixed-precision sequential sampling plan, a weekly sweepnet sampling was conducted at five winter wheat fields in Gorgan region in northern Iran, during two growing seasons of 2006 and 2007. Spatial distribution of the predators was described by fitting data to Poisson (random) distribution, as well as by calculating three dispersion indices. A sequential sampling plan was also developed using the fixed-precision method of Green for estimating the mean density of predators' population. Four species of Coccinella septempunctata L., Propylea quatuordecimpunctata L., Eupeodes corollae (Fabricius) and Episyrphus balteatus (De Geer) comprised 18.75, 14.25, 41.54 and 6.02 percent of the predators' community, respectively. For all predators, with the exception of E. corollae, the parameters b of Taylor’s power law did not differ significantly from one, indicating that populations of them exhibited random spatial distribution. Fitting population’s frequency data to distribution models also showed that Poisson (random) distribution provided a good fit to the population frequencies during most of the wheat growing season. Sequential sampling results showed that the number of sample units required to stop sampling was depended upon mean population abundance and desired level of precision. So that, for C. septempunctata, at density of 0.033-0.6 adult/10 sweepnet and precision level of 0.25, the required sample size ranged from 400 to 41 sweepnets. At the same time, for P. quatuordecimpunctata, at density of 0.02-1.8 adult/10 sweepnets, the required sample size ranged from 775 to 10 sweepnets. For E. corollae and E. balteatus, required sample size ranged from 4 to 400 and 57 to 175 sweepnets, respectively. Regarding number of sample units, at high density of predators, sweepnet sampling was a cost-effective method to estimate predators' population density, whereas at low population levels, sweepnet sampling was a time-consuming method and a quite large sample was required to achieve the desired precision of 0.25. Therefore, we recommend the comparison of precision and efficiency of sweepnet with other sampling procedures to determine the best sampling method for estimating population density of predators in wheat fields.

Keywords: Population fluctuation, Spatial distribution, Sequential sampling, wheat aphids, Predators and Gorgan