Perturbation of regulated apple mites: immigration and pesticide effects on outbreaks of Panonychus ulmi and associated mites (Acari: Tetranychidae, Eriophyidae, Phytoseiidae and Stigmaeidae)

To assess the impact of disturbances on mite populations being regulated at low densities, the phytophagous mites Panonychus ulmi (Koch), Eotetranychus sp., Bryobia rubrioculus Scheuten, and Aculus schlechtendali Nalepa and the predaceous mites Zetzellia mali (Ewing), Amblyseius andersoni Chant, and Typhlodromus pyri Scheuten were sampled during 1996-1997 in 5 apple plots treated with azinphosmethyl (OP) (2 plots), OP + endosulfan (E) (1 plot), or were unsprayed (2 plots) in 1993-1995. P. ulmi had been regulated at very low, but differing densities by predators (OP + E > OP > unsprayed) and the numbers of predator species present were OP + E (1 species), OP (2 species), and unsprayed (3 species) during 1993-1995. During 1996-1997, plots were treated just as in 1993-1995, except that 10 of 20 trees per plot had 150-200 P. ulmi released per tree and all 20 trees of 1 OP and 1 unsprayed plot had esfenvalerate sprayed early in 1996. We expected the peak densities of P. ulmi in 1996-1997 to reflect the earlier densities and the diversities of predators present, lowest peaks and the shortest outbreaks of P. ulmi in 1996-1997 were expected in the unsprayed plots with 3 predator species and the fewest P. ulmi in 1993-1995; highest peaks and longest outbreaks were expected in the OP + E plot with 1 predator species and the most P. ulmi in 1993-1995. Densities in 1996-1997 supported these expectations, but with strong pesticide effects. With no esfenvalerate use, regulation of P. ulmi was as in 1993-1995, but P. ulmi was 10- to 1,000-fold denser with use of esfenvalerate. With esfenvalerate use and release of P. ulmi, they were most dense in year 1 after treatments and the outbreak lasted 2 yr before predators recovered; with esfenvalerate use and no release of P. ulmi, they were the most dense in year 2 and the outbreak lasted only 1 yr. Outbreaks of A. schlechtendali and B. rubrioculus also occurred in year 2 after esfenvalerate use.