Why is it a problem? Prolonged, intense infestations weaken apple trees, seriously affecting the development of shoots, buds and fruit set in the following year.
Where and when is it a problem? Mites can be a pest in most apple growing regions, but serious problems are rare, very localised (even within a farm), and normally a consequence of excessive pesticide use.
Adult red spider mites are about 0.5 mm long and pale red to bright red in colour. They are visible to the unaided eye following careful inspection under leaves. Eggs are minute, spherical, bright red and can occur in sufficient numbers for the bark to appear red.
Initial damage symptoms are light speckling of the leaves. As the mite population increases, feeding causes the leaves to become dull green and later silvery bronze, and leaves are often covered with a fine mould-like webbing. Such leaves are brittle and may drop prematurely. Prolonged infestation weakens the tree. Leaf symptoms are most evident from July to September, but bronzing can occur as early as June under favourable conditions and if populations were high in the preceding year.
Confusion with other pests: The cause of leaf symptoms need to be confirmed by careful inspection. Look for mould-like webbing and red specks under the affected leaves.
Lifecycle: “Winter eggs” can be found on rough bark, axils of the branches, buds and underneath twigs. They hatch from late April to mid-June. On emergence they move to the underside of leaves to commence feeding. “Summer eggs” are paler and laid on the underside of leaves and on twigs. Mites remain on leaves, with moulting castes accumulating around colonies. Development from egg to adult takes about two to four weeks, with five or more generations occurring each year. Breeding ceases towards late autumn with the deposition of winter eggs.
Dispersal: Infestations spread within and between trees by wind. Mites can also be moved around by human activity.
When can damage be expected? Outbreaks are normally associated with the overuse of pesticides. Attacks are particularly severe during hot, dry summers. Heavy rain and wind can strongly reduce mite populations.
Hosts: Fruit tree red spotted mites is known to have a very wide host range. In Bhutan it has only been reported as a problem on apples although it has also been found on pear, plum and peach.
Fruit tree red spider mite is largely considered to be a symptom of pesticide overuse.
General orchard monitoring should include inspection for mite damage which can often be restricted to parts of the orchard.
Effect of variety
Cultivars can differ in susceptibility to the fruit tree red spider mite, but no data is available for Bhutan.
- Normally mites on apples are controlled by natural enemies (especially predatory mites), which should be protected by minimising the use of all pesticides within the orchard.
- Mite outbreaks are generally associated with overuse of pesticides. If they are observed then re-evaluate pest management practices.
- Winter spraying of tree spray oil (TSO) for San Jose scale insects may help in control of mites as well.
- Build-up of this mite is usually associated with indiscriminate and excessive use of pesticides on apple.
- Contact NPPC for advice if outbreaks occur.
Image acknowledgements: NPPC & Bugwood.org