Development of an optimum sampling plan for the population of Aphis gossypii (Hom., Aphididae) on cotton in Greece
A sampling plan was developed for the study of the population dynamics of Aphis gossypii, Glover on cotton. The distribution of the population of A. gossypii within the plant is not randomly distributed in relation to height. At the beginning (end of May June) and at the end (October) of the season, the upper part of the plant is more heavily infested than the lower but this relationship changes in mid-summer (July August). Therefore, the samples for estimating mean population density should be taken proportionally in respect to the height of the plant. The optimum size of the sampling unit was found to be 10 leaves but at the beginning of the season, when the number of leaves per plant is low, a smaller sampling unit could be adopted or even the whole plant can be considered as the sampling unit. The number of samples required to attain a certain precision (i.e. a 10% standard error of the mean) was found to be a strong function of density, fewer samples are required at the higher densities. This is the result of the relationship between the mean and the variance of the population density as it is expressed by the value of the coefficient b in the expression of Taylor's power law. At the low densities, the adoption of the above level of precision leads to a very high number of samples and, therefore, a lower level of precision should be accepted.