Low-density releases of Neoseiulus fallacis provide for rapid dispersal and control of Tetranychus urticae (Acari: Phytoseiidae, Tetranychidae) on apple seedlings.

Releases of Neoseiulus fallacis (Garman) at 1500--6000 per ha when prey were at 0.1-0.3 per leaf provided seasonal control of Tetranychus urticae Koch (all stages) at 1-2 per leaf in an apple seedling rootstock nursery. Predaceous mites (all stages) increased to 0.3-0.4 per leaf after releases and predator prey ratios of < or = 1:3-7 provided pest regulation thereafter. Such low-density releases were thought to be effective because multiple dispersal bouts allowed predators to locate widely distributed spider mites (on 2-6% of leaves). A random-diffusion model simulating predator dispersal (incorporating wind speed and direction parameters) adequately explained movement and pest control patterns. An upright, dense, uniform planting of apple seedlings was an effective producer and recipient for dispersing predators and these attributes seemed to explain why biological control was so effective. Low-density releases of N. fallacis for control of T. urticae are predicted to be less effective on other crops with less prominent profiles and soil coverage.