Estimates of mean population density and spatial distribution of Tetranychus urticae (Acarina: Tetranychidae) and Phytoseiulus persimilis (Acarina: Phytoseiidae) based upon the proportion of empty sampling units.
A simple but general model of density-dependent dispersal in an arthropod population is used to explain the relationship between the density and the spatial distribution of Tetranychus urticae and its predator Phytoseiulus persimilis introduced as a biological control agent on cucumbers in glasshouses in Denmark. Based on the above relationship, estimates of mean mite densities in glasshouses are obtained from sampling data in which only the presence or absence of individuals is recorded. Confidence limits for the true mean density estimated from the proportion of sampling units without individuals (the zero-group) are provided for the 2 mite species. Estimates of variances require that Taylor's power law is a suitable model of the variance/mean relationship. The reliability of density estimates obtaned by the zero-group method is compared with that based on direct counts of individuals occurring in a sample. Although counting provides the most precise estimates for a given sample size, the former method is both easier and faster to use at high population densities. The overall spatial distribution of T. urticae is predicted from an observed proportion of empty sampling units. This presupposes, however, that the negative binomial is a general model of the underlying spatial distribution of the species but not that the parameter k of the negative binomial is independent of the population density.