Apple rust mite overwinters as a fertilised winter female (deutogyne) behind vegetative buds between the bud and the stem of the previous seasons extension growth. The overwintering females often occur in clusters of up to 100 or more in the crescent of hairs immediately behind the bud.
- The mites emerge at bud burst, walking up the leaf stalk of the outer rosette leaves and settling to feed on the leaf lamella, mainly around the base of the leaf.
- Eggs are deposited on the surface of leaves and fruit buds and a generation of males and of summer breeding females (protogynes) develops in May.
- These mites pose the greatest threat to the crop because they and their offspring feed on the receptacles of developing flowers and fruitlets the tender tissue of which is a good food source at this time, as well as on young leaves.
- Feeding on the fruitlets causes russeting damage to fruits (see ‘Damage’ above). Breeding continues throughout the summer, mainly on the undersides of leaves in the extension shoots, there being a number of overlapping generations of summer forms.
- Population growth is rapid particularly in hot conditions which favour the pest.
- Winter females form in the autumn and populations on leaves decline. The females enter hibernation, mainly behind buds in extension shoots.