Olfactometer experiments are used to determine the effect of odours on the behaviour of organisms such as insects or nematodes, and typically result in data comprising many groups of small overdispersed counts. We develop a non-homogeneous Markov chain model for data from olfactometer experiments with parasitoid wasps, and discuss a relation with the Dirichlet-multinomial distribution. We consider the asymptotic relative efficiencies of three different sampling schemes, and give an analysis of data intended to shed light on the effect of previous experience of odours in parasitoid wasps.