Food degradation as a mechanism of intraspecific competition among the larvae of secondary stored product pests

A number of investigators have shown that larval development can be affected by conditioning of the food medium resulting from the accumulation of waste products such as faeces, silk, pheromones, etc. Gordon et al. (1988) found that larvae of the stored product moth Cadra cautella (Walker) were able to continue growing even after the complete exhaustion of their food supply, an effect which the authors attributed to re-use of faecal material deposited in the conditioned medium. In this paper we develop a model describing the growth, development and survival of a cohort of stored product larvae from egg hatch to pupation. Growth and survival rates are assumed to depend only on energy uptake, which in turn depends both on the quantity and quality of the food medium. We compare the predictions of this model with the experimental observations described and modelled by Gordon et al. (1988). At low initial densities our model behaves in a way that is essentially indistinguishable both from the experimental observations and the predictions of their model. However, at high larval densities, where the Gordon et al. model fails to match observed behaviour, our model predicts survival, development times and weights at maturation closely in accord with those observed.