We are interested in the e ect of some types of events on dispersion indexes such as the mean crowding. From a mathematical point of view, we consider that a distribution of individuals is a realization of a point process and we study the consequences of a stochastic point thinning on the model distributional properties. A case study is provided from counting of aphids of the cotton plant carried out before and after thunderstorm. The mortality rate due to the thunderstorm is analyzed according to characteristics of thunderstorms, leaf strata and larvae stages.