Fruit tree red spider mite – additional information

Life cycle

Eggs overwinter on bark and are often most numerous on the smaller branches and spurs.
Hatching begins in late April or May and is complete by mid-June.
The young mites move to the undersides of leaves where they feed and develop.
Five or more generations of mites occur in summer, adult females spreading the infestation as they move to fresh leaves.
The mites form fine silken webs on which they can be carried from tree to tree by wind.
In September, mites begin to deposit winter eggs on bark in response to the arrival of shorter days and cooler temperatures.

Pest status

Fruit tree red spider 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. Fruit tree red spider mite is an infrequent pest of pear.

Other hosts

Also a frequent pest of plum and sometimes cherry. Also occurs on currants, gooseberry, cane fruits, walnut and many wild hosts e.g. Malus, Prunus, Sorbus spp.

Varietal susceptibility

Apple varieties vary considerably in their susceptibility to fruit tree red spider mite. Discovery, Worcester Pearmain and Gala are highly susceptible and are more frequently infested. Cox and Bramley are moderately susceptible.

Distribution

Common and widely distributed and present at least at low levels in most apple orchards.

Recognition

Adult female
Body oval, about 0.4 mm long, strongly convex and a dark red colour. The body is furnished with long setae arising from light coloured pinacula. These whitish spots at the base of the setae are diagnostic for this species.

Adult male
Similar in general appearance to female but smaller, yellow green to bright red and more or less pear-shaped, tapering posteriorly.

Immature stages
Pale yellowish-green to bright red. The first stage larvae has 6 legs

Eggs
Red, roughly spherical, 0.1 mm diameter, with the top drawn to a thin spike Overwintering eggs found on bark (guyed to the bark with threads of silken webbing) in winter and early spring, or on the undersides of leaves during summer.

Other pests or arthropods with which fruit tree red spider mite may be confused

Two-spotted spider mite
The two-spotted spider mite, Tetranychus urticae, is a generally abundant spider mite species which has a very wide host range and does occur on apple and pear trees especially in hot summers.

High populations do not usually develop on apple or pear.
The female is a green colour with two dark patches on the sides of the body.
It causes similar damage to the fruit tree red spider mite.

Bryobia mites
The apple and pear bryobia mite, Bryobia rubrioculus, is superficially similar to the fruit tree red spider mite but is uncommon on sprayed fruit trees.

Female bryobia mites have spatulate setae on their backs.

Tydeiid mites
The tydeiid mite, Tydeus californicus, commonly occurs in small numbers on the undersides of apple leaves usually near the main vein.

It is smaller than fruit tree red spider mite and has a greenish yellow colour and does not cause significant damage.
It provides an alternative food source for the orchard predatory mite Typhlodromus pyri.

Apple rust mite
The apple rust mite, Aculus schlechtendali, causes bronzing damage to the foliage of apple trees similar to that caused by fruit tree red spider mite but the rust mite is much smaller, light brown in colour and wedge-shaped.

The rust mite and fruit tree red spider mite often occur in high numbers together because their key natural enemy, the orchard predatory mite Typhlodromus pyri, is absent.

Damage

Adults and nymphs have needle-like mouth-parts which they use to suck the sap from the surface cells of leaves and, sometimes, young developing fruitlets.

The cells, which have their contents drained, are whitish in colour, causing a light speckling of the foliage.
Later, as populations increase and damage intensifies, the leaves become dull green, brownish and finally, silvery bronze.
Such foliage is brittle and may drop prematurely.
Leaf damage is usually most evident in late summer and attacks are particularly severe in hot, dry summers.
Heavy infestations affect yield and fruit bud formation for the following year.
If large numbers of mites are present in spring, they can infest and feed on the developing fruitlets causing russeting.

Monitoring

If the orchard predatory mite Typhlodromus pyri is established and there is a stable equilibrium between the predator and its prey, detailed monitoring of red spider mite populations is not necessary.

It is prudent to keep an eye out for leaf damage on susceptible varieties (e.g. Discovery, Worcester, Gala).
If the predator/prey equilibrium is not established, then careful monitoring of the pest is necessary, especially during hot weather when populations and damage can increase rapidly.

Overwintering populations

Overwintering populations of eggs on spurs and shoots can be counted in winter.
High numbers indicate high populations were present the previous season and that a balanced equilibrium between pest mites and the orchard predatory mite has not been established.

An average of more than 5 eggs per fruit bud, or the presence of eggs round vegetative buds in the extension growth, indicates a possible problem.
However, the percentage of eggs that hatch successfully is somewhat variable and sometimes damaging infestations do not develop even if high numbers of overwintering eggs are present.

In summer

A sample of at least 25 (preferably 50) expanded leaves should be examined per orchard on each occasion when a pest assessment is done.
The number of leaves with 4 or more mites (motile stages plus eggs) should be counted.
If 7 or more leaves in a sample of 50 leaves have 4 or more mites per leaf (equating to a mean of 2 mites per leaf), treatment with an acaricide is justified, particularly if the orchard predatory mite Typhlodromus pyri is absent, or present in only small numbers.

Assessing numbers of Typhlodromus pyri

When leaves are inspected for numbers of fruit tree red spider mite, the numbers of predatory mites present should be counted at the same time.
The predatory mite often occurs along the main vein, particularly at the point where the main vein joins with a side vein, sheltering in the groove between the veins and the leaf lamella.
The predatory mite is a pale whitish-straw colour, sometimes with red gut when a red spider mite has been consumed and is pear shaped with a smooth body with 17 pairs of setae.
It often moves rapidly over the leaf.

Forecasting

Forecasting methods for fruit tree red spider mite suitable for use by growers 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.

Chemical control

With the exception of spirotetramat, chemicals that are used for control of mites on apple are not systemic. Higher volume spray applications to give good cover are needed to get the best results.

Acequinocyl (Kanemite), clofentezine (Apollo), cyflumetofen (Nealta), fatty acids (Flipper), hexythiazox (Nissorun), spirodiclofen (Envidor) and tebufenpyrad (Masai) are approved for control of fruit tree red spider mite and/or two-spotted spider mite on apples. Spirotetramat (Batavia) is approved for control of sucking insect pests.
Clofentezine (Apollo) and hexythiazox (Nissorun) are ovicides. If significant populations of overwintering eggs are present, a spray may be applied up to just before blossom before egg hatch commences.
It is important to use a high spray volume to thoroughly wet the bark as only eggs that are directly intercepted by spray are likely to be controlled.
Application is best made in the late dormant period before green cluster as it is easier to get good spray cover before the rosette leaves develop.
For damaging summer infestations, acequinocyl (Kanemite), cyflumetofen (Nealta), fatty acids (Flipper), spirodiclofen (Envidor) or tebufenpyrad (Masai) are the available choices.
The product chosen will depend on other pests to be controlled and the history of previous use of acaricides (see below).
These acaricides are best applied when most eggs have hatched but before breeding infestations become established.
Higher volume sprays to achieve good cover are more effective. Tebufenpyrad (Masai) or spirodiclofen (Envidor) should not be applied before 90% petal fall and after bee activity has ceased because of the high risk to bees.
Acequinocyl (Kanemite) and cyflumetofen (Nealta) are new products and their effect on bees is unknown. Until experience is gained, it is best to wait until 90% petal fall before making an application.
A full approval for spirotetramat (Batavia) on apples for the control of sucking insect pests will control fruit tree red spider mite, but growers may prefer to reserve its use for more difficult to control pests such as woolly aphid or rosy apple aphid. It must be applied after flowering and works best when pests are moving from brown wood to green tissue. It will prevent population build-up but does not offer pest ‘knockdown’.
The bioinsecticide fatty acids (Flipper) has an EAMU approval for use on apples. It is effective at controlling sucking insect pests such as aphids, whitefly and mites, so is likely to control fruit tree red spider mite. It is known to complement the use of Batavia as it provides quick ‘knockdown’. Its safety to beneficial insects such as Typhlodromus pyri and the parastic wasp Platygaster demades is unknown, but it is generally safe to many other predators and parasitoids, so is considered to be more suitable to IPDM programmes than the synthetic pyrethroids.

Resistance to acaricides and avoiding its development

The fruit tree red spider mite readily develops strains resistant to insecticides and acaricides.
The best way to avoid the development of resistance is to ensure that the orchard predatory mite Typhlodromus pyri is conserved so that acaricide use is unnecessary.
Acaricides should be used as little as possible, alternating different products to reduce the risk of development of resistant strains.

Cultural and biological 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 fruit tree red spider mite and often apple 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.

Natural enemies

The orchard predatory mite Typhlodromus pyri

Several species of predatory mite in the family Phytoseidae prey on fruit tree red spider mite, but Typhlodromus pyri is the species which occurs in selectively sprayed apple orchards and is the key natural enemy of fruit tree red spider mite because it has developed resistance to organophosphorus insecticides.
The predatory mite is also the key natural enemy of apple rust mite.
If populations of the mite are conserved by avoiding the use of harmful insecticides, notably synthetic pyrethroids, fruit tree red spider mite and apple rust mite are seldom a problem.

Other predators

Fruit tree red spider mite is preyed on by several other generalist insect and mite predators such as anthocorids, mirids and spiders and the stigmaeid mite Zetzellia mali.

Biological control

Establishment of the orchard predatory mite, Typhlodromus pyri, is crucial. Unless the predator is established, regular outbreaks of fruit tree red spider mite and apple rust 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, fruit tree red spider mite and apple rust 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 present and can develop rapidly on newly planted trees which do not have established populations of the predatory mite (often because they have been sprayed with predator-harmful pesticides in the nursery).
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.

Biotechnological control

Biotechnological control methods have not been developed for fruit tree red spider mite.

Apple Best Practice Guide