Sequential sampling of Rhopalosiphum maidis and Schizaphis graminum (Hem.: Aphididae) in wheat fields of Badjgah, Fars province, Iran

Between 2010 and 2011, spatial distribution and fixed precision sequential sampling plans were determined for two aphids, Rhopalosiphum maidis F. and Schizaphis graminum R., in wheat fields in Badjgah countryside of Iranian Fars province. For this purpose, two fields of two hectares each were selected and sampled on weekly basis. Each sample consisted of 200 randomly selected wheat plants along with the total number of counted aphids. Based on RV, the best sample unit was four stems. This data was used to describe spatial distribution pattern of R. maidis and S. graminum by Taylor’s power law (TPL) and Iwao’s patchiness regression methods. The results indicated aggregated spatial distribution of aphid's populations in wheat fields, based on the mentioned methods. The TPL provided a better description of the aphid's spatial distribution. Since regression line slopes were not significantly different for S. graminum and R. maidis, the same Green model was proposed for both species. In this model, minimum numbers of samples were 20, 12 and 9 for precision levels of 0.15, 0.25 and 0.3, respectively. Comparing Green and conventional methods in 0.15, 0.25 and 0.3 precision levels showed that the number of required samples were reduced 77.8 ± 1.05, 78.3 ± 0.91 and 81.4 ± 0.81 percent, respectively. Based on Wilson and Room's model, when the mean populations of aphids were 1.6, 3.86 and 5.62, the proportion of infestation in the field were 0.5, 0.75 and 0.85, respectively. Therefore, by increasing the infestation percentage of the samples, the number of required samples will be reduced.