Simulating effects of environmental factors on biological control of Tetranychus urticae by Typhlodromus pyri in apple orchards John Michael Hardman • Wopke van der Werf • Suzanne E. Blatt • Jeffrey L. Franklin • Richard Karsten • Holger Teismann Received: 18 April 2012 / Accepted: 20 November 2012 Ó Her Majesty the Queen in Right of Canada 2013 Abstract Successful biological control of mites is possible under various conditions, and identifying what are the requirements for robust control poses a challenge because inter- acting factors are involved. Process-based modeling can help to explore these interactions and identify under which conditions biological control is likely, and when not. Here, we present a process-based model for population interactions between the phytophagous mite, Tetranychus urticae, and its predator, Typhlodromus pyri, on apple trees. Temperature and leaf nitrogen concentration influence T. urticae rates of development and reproduction, while temperature and rate of ingestion of prey and pollen influence T. pyri rates of survival and reproduction. Predator and prey population dynamics are linked through a stage structured functional response model that accounts for spatial heterogeneity in population density throughout the trees. T. urticae biomass-days (BMD’s), which account for sizes of larvae, nymphs and adults, indicate level of mite-induced leaf damage. When BMD’s exceed 290 per leaf, there are economic losses. When BMD’s exceed 350 per leaf, T. urticae population growth is curbed and eventually the population decreases. Simula- tions were run to determine which conditions would lead to current year economic loss and increased risk of loss in the following year, i.e. where more T. urticae than T. pyri are present at the end of September. Risk was high with one or more of the following initial John Michael Hardman is retired. Electronic supplementary material The online version of this article (doi:10.1007/s10493-012-9640-z) contains supplementary material, which is available to authorized users. J. M. Hardman  S. E. Blatt (&)  J. L. Franklin Atlantic Food and Horticulture Research Centre, Agriculture and Agri-Food Canada, 32 Main Street, Kentville, NS B4N 1J5, Canada e-mail: Suzanne.Blatt@agr.gc.ca W. van der Werf Chair Group Crop and Weed Ecology, Centre for Crop Systems Analysis, Department of Plant Sciences, Wageningen University, P. O. Box 430, 6700 AK Wageningen, The Netherlands R. Karsten  H. Teismann Department of Mathematics and Statistics, Acadia University, Wolfville, NS, Canada 123 Exp Appl Acarol DOI 10.1007/s10493-012-9640-z