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Apple rust mite – additional information

Life cycle

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.

 

Pest status

Apple rust mite is an important secondary pest of apple which is much more troublesome if its key natural enemy, the orchard predatory mite Typhlodromus pyri, is not present.

A similar species, the pear rust mite, Epitrimerus piri, is frequently an important pest of pear. Pear trees have smooth, hairless leaves and less prominent leaf veins than apple. They do not favour predatory mites, which are not present in significant numbers in commercial pear orchards. Outbreaks of pear rust mite are frequent on pear, almost an annual event, for this reason.

 

Other hosts

Both rust mite species can breed on a range of other rosaceous trees and shrubs but are usually only numerous on their main host.

 

Varietal susceptibility

All apple varieties are susceptible to apple rust mite, but Bramley is highly susceptible.

 

Distribution

Apple rust mite is very widespread and is probably present to a greater or lesser extent on practically all apple trees.

 

Natural enemies

The orchard predatory mite Typhlodromus pyri
Several species of predatory mite in the family Phytoseidae prey on apple rust mite, but Typhlodromus pyri is the species which occurs in selectively sprayed apple orchards and is the key natural enemy of apple rust mite because it has developed resistance to organophosphorus insecticides.

  • This predatory mite is also the key natural enemy of the fruit tree red spider mite.
  • If populations of the mite are conserved by avoiding the use of harmful insecticides, notably synthetic pyrethroids, apple rust mite and fruit tree red spider mite are seldom a problem.

Other predators
Rust mites are probably preyed on by several other generalist insect and mite predators such as anthocorids, mirids and the stigmaeid mite Zetzellia mali.

 

Monitoring

Overwintering populations
The number of apple rust mite overwintering behind buds in the previous season’s extension growth should be determined in winter in orchards where a satisfactory and stable balance between the mite and the orchard predatory mite has not been established.

  • A sample of at least 20 one-year extension growths should be sampled and the number of rust mites behind three buds in each shoot, one bud towards the apex, one in the middle and one at the base of the shoot, should be counted and recorded.
  • This is most easily and comfortably done indoors using a microscope but the shoots need to be stored in a fridge until the counting is done.
  • Alternatively, it can be done in the field, bending the shoot at the point of the bud so the bark behind the bud can be examined with a hand lens.
  • The average number of rust mites overwintering per bud should be calculated.
  • If the mean number of mites exceeds 10 per bud, there is a significant risk of fruit damage.

At green cluster
Examine the undersides of the outer rosette leaves, especially round the bases using a hand lens and count the number of rust mites present.

  • Holding the leaf up to the light so more light shines through the leaf makes the mites more easily visible but care must be taken not to look directly at the sun as this could cause damage to eyes.
  • At least 25 leaves (preferably 50) should be examined per orchard.
  • An average of 5 or more mites per outer rosette leaf indicates a significant threat of damage to fruitlets.

During blossom and early fruit development (cell division)
Examine the rosette leaves as above and, importantly, also the receptacles of the flowers and the young developing fruitlets for the presence of mites.

  • The mites are often found near the point where the fruit meets the calyx and this is where examination should be concentrated.
  • If an average of one or more mites is found per flower or fruitlet, it is likely that some russeting damage will be caused.

In summer
More mature fruits are not susceptible to apple rust mite as their skin is less palatable. Mites occur on the undersides of leaves in extension shoots.

  • The undersides of leaves should be examined to ensure they remain fresh and green.
  • If any bronzing is present, leaves should be examined more closely for rust mites.
  • If large populations are present and significant bronzing is being caused, treatment may be justified.

Forecasting

Forecasting methods for apple rust mite have not been developed. However, population increase can be very rapid in hot weather and bronzing damage to the undersides of leaves can intensify rapidly.

 

Damage

Adults and nymphs have needle-like mouthparts which they use to suck the sap from the surface cells of leaves and young developing fruitlets. Three types of damage can be distinguished.

Damage to rosette leaves caused at and shortly after bud burst
Young rosette leaves are inundated with mites emerging from behind buds which feed on the leaf tissue causing it to become dull, puckered and shrivelled.

  • The outermost leaves are the worst affected, particularly around the base.

Damage to fruitlets
Mites feed on the receptacle of young developing fruitlets mainly round the calyx, swarming down the cheek when numbers are large.

  • The feeding results in russeting which occurs in an irregular ring round the calyx and on the cheek of the fruit.
  • The damage can be very severe if populations are high.
  • The damage can be confused with ‘frost eye’ a ring of severe russeting round the calyx caused by frosts during or shortly after blossom. However, frost eye occurs as a clearly defined ring round the calyx.

Damage to leaves in extension shoots
Mites feed on the undersides of leaves, mainly in the younger leaves of extension shoots which are fresh and green.

  • The undersides of leaves become brown (bronzed).
  • As populations increase and the older leaves are less palatable to mites, so the infestation tends to move up the shoots with the growth.

 

Chemical control

Chemicals that are used for control of mites on apple and pear are not systemic. Higher volume spray applications to give good cover are needed to get the best results.

  • Diflubenzuron (Dimilin) is specifically recommended for control of apple rust mite. Though selective, it is only partially effective and is unlikely to control severe infestations adequately.
  • A programme of sprays of sulphur (various products) at reduced rates (3-5 kg a.i. / ha), applied to control mildew on apple, will suppress rust mite and fruit tree red spider mite.
  • However, multiple sprays of sulphur are likely to be harmful to the orchard predatory mite, so such an approach is not ideal.
  • Some apple varieties are sulphur shy (consult the label for details) but are often safe at low rates.
  • Clofentezine (Apollo) and tebufenpyrad (Masai) are approved for control of fruit tree red spider mite on apple and are not specifically recommended for control of apple rust mite.
  • When applied for control of fruit tree red spider mite, they may give partial control of rust mite but should not be relied on to control damaging infestations.
  • Spirodiclofen (Envidor) is specifically recommended for control of apple rust mite. Young larval stages are most susceptible.

 

Biological control

Establishment of the orchard predatory mite, Typhlodromus pyri, is crucial. Unless the predator is established, regular outbreaks of apple rust mite and fruit tree red spider mite are inevitable and these can be very damaging and difficult and costly to control.

  • Once the predator is established and the biological equilibrium between the predatory mite and the pest mite has stabilised, rust mite and fruit tree red spider mite seldom cause problems, providing the equilibrium is not disturbed by the use of pesticides harmful to the predatory mite.
  • The predatory mite will establish naturally in apple orchards but this can be a slow process.
  • Pest mite infestations are often present and can develop rapidly on newly planted trees which do not have established populations the predatory mite (often because they have been sprayed with predator-harmful pesticides in the nursery e.g. with frequent sprays of fungicides to control canker).
  • Where the orchard predatory mite is absent, e.g. in newly planted orchards, it should be introduced in summer by transferring extension shoots from established orchards where the predatory mite is abundant.
  • Summer prunings may be used. Ideally, at least one shoot should be placed amongst the foliage in each tree of the orchard where the predator is to be introduced.

 

Cultural control

Young trees from the nursery used to plant new orchards are often infested with apple rust mite and/or fruit tree red spider mite and do not have established populations of the orchard predatory mite Typhlodromus pyri.

  • Damage by rust mite can occur rapidly in the first season after planting whereas the pest may be unimportant in other established orchards on the farm.
  • Steps should be taken to ensure the nursery trees are not heavily infested with pest mites before they are purchased and delivered to the farm.
  • In orchards, hot, dry situations favour rapid population increase of apple rust mite.
  • Overall bare soil orchards should be avoided.

 

Biotechnological control

Biotechnological control methods have not been developed for apple rust mite.

 

Recognition

Adults
Minute, 0.16-0.18mm long, wedge-shaped mites, yellowish-brown in colour, with two pairs of legs, each leg terminating in a branched feather-claw. Body annulated and with a distinct dorsal shield bearing a pair of setae on the hind margin. Two forms exist – the summer form (protogyne) and the winter form (deutogyne). Morphological differences between the forms can only be distinguished with the aid of a good microscope.

Nymphs
Similar to adults but much smaller.

Eggs
Minute, oval and translucent. Only visible with a microscope.

Other pests with which the pest may be confused

Fruit tree red spider mite also causes bronzing of leaves and often occurs in association with apple rust mite because insufficient predatory mites are present to regulate pest mite populations. However, the fruit tree red spider mite has a completely different appearance to the apple rust mite.

 

Further reading

Easterbrook, M. A. 1996. Damage and control of eriophyid mites in apple and pear. In:  Eriophyid mites, their biology, natural enemies and control.

Lindquist, E. E., Sabelsi, M. W. & Bruin, J. (Eds).  World Crop Pests Volume 6, Elsevier, Amsterdam, 527-541.