Aculus schlechtendali (Nalepa)
Taxonomic placing: Acari, Prostigmata, Eriophyoidea Eriophyidae.
Common name: Apple rust mite (ARM).
Morphology: The body of the protogyne is yellow-orange, about 165-180 microns long; deutogynes are somewhat smaller. The prodorsal shield is subtriangular, with an acute anterior lobe over the gnathosoma. The featherclaw is 4-rayed.
Distribution: In most regions where apples grow.
Host plants: Apple and Malus floribunda L.
Life history: ARM is vagrant on lower leaf surfaces. Its deutogynes overwinter in and around dormant buds, producing about 20 eggs/female. In spring their protogyne descendants, which lay 60-70 eggs/female, invade the developing flowers and fruit buds. Later they move to the leaves, on which they raise several generations and very large populations; up to several thousand/leaf. At that time a life cycle may be completed in 1-2 weeks. By mid-late summer, as the foliage turns rusty-brown, pest numbers decline and the protogynes appear.
Economic importance: Mite feeding on young leaves causes rusting, pitting, browning, curling, some drop, and thus reduced photosynthesis and yield. On developing pomes damage includes russeting, some cracking and sometimes changes in coloration, and earlier ripening, depending on the cultivar. The size, fresh weight of infested fruits and their soluble sugar content are reduced, affecting their taste. Longer periods of infestation result in fewer flower buds and in reduced growth. Damage may occur when there are >200 motile mite/leaf, being more intense in nurseries and on young trees. When ARM and Panonychus ulmi co-occur on apple leaves, the former may displace P. ulmi, probably due to changes in the foliage.
Cultivar tolerance: In Turkey the cultivar “Golden Delicious” appears to be less affected by the mite than ““Starking Delicious”.
Chemical control: Abamectin, carbamates, as well as mineral oils provided good ARM control, especially if applied prior to blooming. On the other hand, applications of pyrethroids and of the insecticide DDT induced mite outbreaks, possibly due to killing natural enemies. Fungicides can also reduce ARM populations, often without harming their predators.
Biological control: Several phytoseiids and Zetzellia mali reduce and at times control ARM populations in different regions. In Northern Ireland the mite Anystis baccarum (L.) (Anystidae) significantly reduced ARM numbers. In Turkey 2 phytoseiids, Z. mali, a tydeid and 14 Coccinellidae were recognized as natural enemies of ARM. Species of Orius also feed voraciously on ARM. In addition, the fungus Paecilomyces lilacinus (Thom) Samson caused about 90% mortality in some pest populations in Turkey.
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