Comparison of Sampling Plans for Roes-Grain Aphid, Metopolophium dirhodum (Walker) (Hem.: Aphididae) Population on Wheat

In order to estimate the rose-grain aphid, Metopolophium dirhodum (Walker), population in wheat fields, optimum sampling plans were calculated for fixed precision levels recommended for research (0.10) and integrated pest management (0.25) purposes using the general formula, Taylor’s power law and Iwao’s patchiness regression methods. Weekly counts were carried out from the end of March or early April (beginning of aphid migration) until the breakdown of the population in the field (toward the end of May) in Varamin region (near Tehran, capital of Iran) during 1999 and 2000 growing seasons. In each sampling occasion, 50 tillers were randomly chosen and all leaves were examined for the presence of M. dirhodum individuals. Based on the results, aphid population density was low at the beginning of the season, but it increased rapidly in late April in two studied years. Population peak of rose-grain aphid was observed in late April and it then decreased rapidly, so that there were low aphid individuals per tiller in early May. Sample size curves obtained using three methods studied were similar and indicated that estimating rose-grain aphid population in wheat can be accomplished with relatively little sampling effort, when mean population density was higher than two aphids per tiller. However, sample size increased dramatically for M. dirhodum, when mean population was below 0.2 aphids per tiller. The optimum number of samples decreased as population density increased. The calculated values for the optimum number of samples indicated that a precision level of 0.10 is not practically feasible for the densities observed in the present study. However, the calculated values of the optimum number of samples at precision level of 0.25 provided a basis for the sampling program, so that the estimate’s precision for various population parameters would be at acceptable level.