Spatial distribution of aphid on cotton and sample size

Abstract

This research was performed with the aim to study aphid, Aphis gossypii Glover (Hemiptera: Aphididae), spatial distribution as well as establish the sampling number to be used on IPM program. On the first area the BRS Aroeira cultivar was planted in 20 isolated, 2 x 2m, plots containing up to 16 plants. On these plots the number of aphid colonies was counted on eleven whole plants, at a weekly basis. On the second area the same cotton cultivar was used on 21 contiguous plots of five rows of 5m with the exception of the tree last ones of four meters long. In this study evaluation was done using a score scale on the upper tree completely expanded leaves. The following dispersion indexes were used: variance/mean ratio (I), Morisita Index (I_δ), Green Coefficient (Cx) and k exponent of the negative binomial distribution. Based on these indexes it was observed that aphid followed the aggregation distribution patterns. The mathematical model that best describe this distribution, for aphid was the negative binomial. The sample number suggested to be used in IPM was fifty using as a sampling unit the upper first completely expanded leaf.