Under mild infestation, light bronze speckles appear on the leaves along the main veins. As the population of mites grows, the speckles, that are the results of the mite's sucking activity, can spread to the whole leaf. Leaves can curl upwards and the foliage takes a bronzed or rust-brown discoloration. The damage to leaves and buds reduces the photosynthetic activity of the tree and results in poor shoot growth, insufficient development of wood, poor ripening of fruits or premature drop. This increases the vulnerability of shoots to winter frost and reduces flowering in the following season.
The symptoms are caused by the feeding activity of the European red mite (Panonychus ulmi), which can infest a large number of pome and stone fruit, as well as grapevines. Males are yellowish red, tinged with two red spots on the back and about 0.30 mm long. Females are slightly longer (0.35 mm) and more oval than males. They are characterized by a brick red body and strong white hairs protruding from pearly spots on the back. They lay red eggs principally on bark crevices, fruit calix or dormant buds in late summer and on the underside of leaves during the spring. The number of generation per year is regulated by temperatures and food supply and can range from 2-3 in cool weather to 8 in warmer climates. An oversupply of nitrogen stimulates plant grow and favors the pest. Wind and rain, in turn, increase the mortality of the insects.
Biological control through predatory mites works well on fruit tree orchards. Further natural antagonists include flower bugs, ladybugs, some varieties of capsid bug, as well as glassy-winged mirid bug (Hyaliodes vitripennis) or Stethorus punctum. Approved narrow range oils can also be used.
Always consider an integrated approach with preventive measures together with biological treatments if available. If thresholds are exceeded, acaricides or miticides can be applied if clusters with red eggs are found on shoot tips in winter. In general, try to keep the application of chemical treatments to a minimum. These can affect the populations of beneficial insects and trigger the resistance of some populations of mites. Horticultural mineral oil can also be used to reduce their numbers.
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