The eriophyid of apple (Aculus schlechtendali): morphology, biology and damage.
The systematics, morphology, biology, injuriousness and integrated control of Aculus schlechtendali, a pest of apple and pear, are described. This eriophyid constitutes an alternative prey for those species of phytoseiids that have a role in the natural biological control of the tetranychid Panonychus ulmi. Estimations of the numerical thresholds of the mite which can be tolerated by apple trees for the months of May, June, July and August are given; apple trees can register a population greater than 200 mites per leaf before the first symptoms of damage are evident. Phytoseiid and stigmaeid predators play a primarily role in the biological control of A. schlechtendali, while insect predators are of secondary importance. Sudden and widespread infestations of the pest can be caused by an imbalance in the biological equilibrium between predators and prey, often following pesticide use. In Trentino, Italy, chemical control of the pest is mainly limited to cv. Golden Delicious; up to 5 treatments are applied per year, usually in the post-floral phase.