Equilibrium Densities of European Red Mite (Acari: Tetranychidae) after Exposure to Three Levels of Predaceous Mite Diversity on Apple

Abstract

Prey and predator mites were sampled in 1994-1995 in apple plots that received similar horticulture but the following pesticide treatments in 1990-1995: (1) azinphosmethyl (an organophosphate) + endosulfan (OP + E), (2) azinphosmethyl alone (OP), and (3) unsprayed (1993-1995). Plots supported different levels and diversity of phytophagous mites including the OP-resistant European red mite, Panonychus ulmi (Koch), and 3 predaceous mites - OP-resistant Typhlodromus pyri Scheuten, OP-tolerant Zetzellia mali (Ewing), and OP-susceptible Ambhyseius andersoni (Chant). We expected that more predator diversity would result in regulation of prey at lower equilibrium densities than would less predator diversity. OP + E had the most P. ulmi (but 200 times less than at the beginning of the experiment in 1990) and the most T. pyri, OP had fewer P. ulmi and T. pyri but the most Z. mali, and unsprayed had the fewest P. uhni and T. pyri, moderate Z. mali, and the most A. andersoni. Total predator density did not differ among the treatments in either 1994 or 1995. Data supported the general hypothesis that regulation of P. ulmi occurred at the lowest equilibrium levels in plots with the most diversity of predators (unsprayed) and at the highest levels in plots with the least diversity (OP + E). OP was intermediate for both. Two other mites, OP-susceptible Eotetranychus sp. and OP-resistant Aculus schlechtendali Nalepa, had low and more similar levels across plots than P. ulmi. Bryobia rubrioculus Scheuten occurred only in plots that were unsprayed because of pesticide susceptibility. Roles of each predator and pest mite in the apple mite complex are discussed.