Modelling the Effects of Plant Species on Biocontrol Effectiveness in Ornamental Nursery Crops

1. The development of biological control strategies for ornamental crops is challenging due to the high diversity of crops grown and zero tolerance to pest presence. In an attempt to provide a biological basis for the development of biological control, an individual-based stochastic modelling approach was developed. 2. This approach has been used to examine the impact of tritrophic interactions on biological control of the spider mite Tetranychus urticae by the predatory mite Phytoseiulus persimilis. The effect of differences in the biology of T. urticae and P. persimilis on three nursery stock crops, Ceanothus thyrsiflorus var. 'Autumnal Blue', Choisya ternata and Euonymus japonicus, was investigated through simulation experiments. 3. The effect of pest aggregation and frequency of predator introduction on the control of T. urticae was examined at a range of initial pest and predator densities. Control was assumed to occur when T. urticae was driven to extinction, due to zero tolerance to pest presence on these ornamental crops. 4. 'One-off' introductions of P. persimilis rarely caused T. urticae to become extinct, but repeated weekly introductions always led to pest extinction, regardless of the plant species, at the highest introduction rate of 1000 predators (10 stem-1). Time to pest extinction was similar on all plant species with repeated introductions, but was influenced by differences in the biology of pest and predator on the three plant species. 5. Increased aggregation of T. urticae led to greater extinction at lower predator introduction rates, but time to extinction was increased due to the time taken for the predator to cope with the increased pest density. 6. These predictions highlight the importance of plant species and spatial dynamics of both pest and predator in determining the efficiency with which biological control can be achieved on ornamental crops, suggesting that prophylactic application of predatory mites may be preferable to reactive application.