How width of the herbicide strip affects mite dynamics in apple orchards

Wide herbicide strips in apple orchards promote growth and yield but may induce soil erosion, leaching of nitrates into ground water, and increases in two-spotted spider mites, Tetranychus urticae Koch. In this study we monitored mite dynamics in apple trees and used sticky bands on tree trunks to track T. urticae immigration into Nova Spy apple trees in plots with wide (2 m) or narrow (0.5 m) herbicide strips. Concentrations of leaf N in trees were higher with wide herbicide strips, which would increase fecundity and population growth of T. urticae and the European red mite, Panonychus ulmi (Koch). Also, there were higher rates of T. urticae immigration from the ground cover vegetation into the trees. In 2006, and for most of 2007, densities of T. urticae, but not P. ulmi, were higher with wide herbicide strips. However, by late summer 2007, densities of both tetranychids were lower with wide herbicide strips. This is because both risk factors were counterbalanced by higher densities of phytoseiid predators, mostly Typhlodromus pyri Scheuten. In 2006, ratios of phytoseiids to tetranychids were usually several-fold lower with wide herbicide strips, but by late summer 2007 predator-prey ratios were actually higher with wide strips. However, this increase in phytoseiids density can only occur where the pesticide program is not too harsh. Hence the impact of width of herbicide strip on mite dynamics is strongly affected by the composition and size of the phytoseiid population and the impact of pesticides on predation.