Bioassays with natural mortality: handling overdispersion using random effects

Abstract

In fitting dose-response models to entomological data it is often necessary to take account of natural mortality and/or overdispersion. The standard approach to handle natural mortality is to use Abbott's formula (Abbott, 1925), which allows for a constant underlying mortality rate. Standard overdispersion models include beta-binomial models, logistic-normal, and discrete mixtures. We extend the standard model (Morgan, 1992), and include a random effect in the dose levels, using the approach described in Aitkin et al. (2009). We consider the application of this model to data from an experiment on the use of a virus (PhopGV) for the biological control of worm larvae (Phthorimaea operculella) in potatoes, using a procedure implemented in software R. Using the model with random effects in the dose levels, we obtained a better fit than that provided by the standard model.