The slug Deroceras reticulatum is a widespread pest of many agricultural and horticultural crops. The slug can be controlled with the commercially available nematode parasite, Phasmarhabditis hermaphrodita. Because the nematode is expensive it is vital to optimise application dose rates and timing. We used two previously published descriptive models to derive a predictive model that estimates the reduction in slug damage caused by individual nematode doses over time, in relation to slug density. The model was tested in a series of independent mini-plot experiments comprising a wide range of nematode and slug densities and was found to predict reductions in slug damage with a high degree of correlation and no significance of lack of fit over all slug and nematode densities. A key feature of the model application is that nematode rates are based on slug numbers per unit area, rather than unit area alone. The model has much potential to be used in selecting the appropriate nematode application rate and timing to control Deroceras reticulatum damage in a range of crops. The model also highlights the importance of accurate estimates of slug population density together with predictive modelling of slug population dynamics to exploit the full potential of the model for optimising the use of P. hermaphrodita for slug control.