Model Simulating the Use of Miticides to Control European Red Mite (Acarina: Tetranychidae) in Nova Scotia Apple Orchards

A model was developed to simulate chemical control of the European red mite, Panonychus ulmi (Koch), in Nova Scotia apple orchards. Simulated densities of summer eggs and motile P. ulmi were similar to densities observed in experimental plots that had been treated with miticide on different dates. The P. ulmi model was used to estimate the best dates to apply miticides with different biological half-lives (7 or 30 d) and specific toxicities (i.e., toxic to all eggs; toxic only to summer eggs; toxic only to motile forms). Criteria of effectiveness of a given application were cumulative mite-days to 15 July, yield per tree, and density of P. ulmi winter eggs on 31 August. The best dates to apply miticide (usually shortly before or after winter eggs hatched) differed according to the characteristics of the miticide and the criterion of effectiveness (e.g., the best dates for preserving yield were not necessarily the best dates for reducing winter eggs). The model also indicated problems in the use of the current economic threshold based on counts of summer eggs and motile P. ulmi on leaves. If the initial P. ulmi population is moderate or high and the choice of application date is based on mite density on leaves, then application of miticide may be too late to prevent significant yield lose sin the current year, a high population of winter eggs at the end of the season, and reduced return bloom and yield the following year. The model suggests the importance of early spring assessments of winter egg density and the necessity of considering age structure of the P. ulmi population and characteristics of the miticide in selecting dates for miticide application.