Apple leaf midge – additional information

Life history

Eggs are laid amongst hairs in the youngest tiny leaves in shoot tips, often in large numbers. These hatch in 3-5 days and the larvae feed on the upper epidermis causing the margins to curl and roll round themselves.

Larvae are full-grown in 2-3 weeks and then drop to the ground to pupate in silken cocoons in the soil. Lack of rain may delay the exit of the full-grown larvae from the hardened leaves and hence lengthen the development period. Sometimes, a mass exodus from the leaf rolls by large numbers of larvae occurs when a long period of dry weather is terminated by a thunder storm.

There are typically three generations per year, larvae of the third generation overwintering in cocoons in the soil before pupating in the spring and emerging in late April or May, often during or shortly after bloom.

Recognition

Adult
Very small, 1.5-2.5 mm long, delicate fly, with broad, slightly hairy, wings with little venation. Antennae long, beaded with whorls of hairs in male, plainer in females. Females have a red abdomen. Usually seen resting or ovipositing in the shoot tips of apple.

Larvae
2.5-3 mm long, maggot-like larvae. Translucent-white when young but becoming pink or orange when mature. Dorso-ventrally flattened with a bi-lobed sternal spatula (‘breast bone’). Found in the tight leaf rolls they cause.

Eggs
Tiny, cigar–shaped and reddish or brown. Found amongst hairs in the very youngest leaves in shoot tips.

Other pests with which the pest may be confused

Tortrix moth caterpillars also cause leaf rolls but tortrix caterpillars cannot be confused with apple leaf midge larvae.

Natural enemies and biological control

Natural enemies

Predators
Birds, probably tits, are reported to predate larvae, causing incidental damage to the leaf rolls. Predatory anthocorids and mirids are important natural enemies of apple leaf midge larvae in leaf rolls. It is probable that larvae and cocoons in the soil are preyed upon by predatory ground beetles.

Parasitic wasps
The parasitic wasp Platygaster demades is the most important natural enemy of apple leaf midge. The adult wasp is minute and lays its eggs in those of the midge. The occurrence of the adult parasite is closely synchronised with its host.

Usually one, sometimes two but up to three or four eggs may be deposited in one host egg. The parasite does not hatch until after the egg of its host has hatched.
The tiny globular parasite larvae are found in the anterior end of the larvae, attached possibly to the salivary glands. Development is slow until the host larva is fully-grown.
The life cycle of the parasite is poorly understood but there are at least two generations, possibly three or four per annum.
High levels (>80%) of parasitism by Platygaster demades have been shown to occur in Integrated Pest Management in other European countries and the parasite is considered to be the key natural enemy of the leaf midge.
The adult parasite is believed to be sensitive to broad-spectrum insecticides. Leaf midge populations have been shown to decline where broad-spectrum insecticides are not used and the parasite is allowed to establish.

Another larval parasite, Torymus chlomerus, also occurs occasionally. It is an ectoparasite, which in the larval state lives outside but next to its host.

The larvae are distinctive and sparsely clothed with spines. They usually occur singly in midge leaf rolls.
One Torymus larva can consume 2-5 leaf midge larvae (and could be considered to be a predator).

Biological control

In some young and established orchards, notably Bramley, severe outbreaks of leaf midge occur from time to time, greatly reducing the area of photosynthetic foliage and probably reducing fruit size and fruit bud formation. These outbreaks occur because there are no suitable chemical control methods for apple leaf midge at present in the UK and because the parasite that regulates populations of the midge, Platygaster demades, is absent or at low levels only in most commercial apple orchards because of indiscriminate spraying with broad-spectrum insecticides.

In order to establish the natural balance, the priority should be to establish and increase the parasite Platygaster demades. The parasite occurs naturally in the UK.

The adult, which has a distinctively shaped abdomen and wing venation, can sometimes be seen in the shoot tips of apple amongst clusters of leaf midge eggs. However, accurate identification is for the specialist entomologist as there are many species of parasitic wasp of similar appearance.

The degree of parasitism of the population is best determined by dissecting mature larvae. Each larva is immersed in a small pool of water in a watch glass under a binocular microscope.

A pair of dissecting needles are drawn in opposite directions across the anterior (head end) of the larva causing the body to rupture and any parasite larvae to burst out into the water along with some of the body contents.
The larvae are globular and there may be one, two or several inside a midge larva.
Dissection is best done in a laboratory by an experienced entomologist.

Where the parasite is totally absent from an orchard it may be possible to introduce it from another orchard where it is abundant. This was done in an experimental orchard where the parasite was absent at East Malling in the 1990s.

Leaf rolls containing mature (pink-orange) leaf midge larvae, 25% of which were parasitised by Platygaster demades, were collected from an unsprayed orchard at Marden in summer and one leaf roll was lodged in the foliage of the orchard where the parasite was to be introduced. The parasite was present at low levels the following year and gradually increased in subsequent years. However, it was not possible to determine whether the parasite population developed from the one introduced.
In the Netherlands, the parasitoid has been found to greatly reduce leaf midge populations, though irregular flare-ups of the midge have been found to occur. These are followed by an increase in the parasitoid which then reduces populations of the pest.

The adult parasite is believed to be particularly sensitive to broad-spectrum insecticides.

Their use while the adult parasite is active, i.e. during the egg-laying period of the leaf midge, should be avoided.
Egg laying by the midge should be monitored closely. Use of broad-spectrum insecticides should be avoided for a week or two when eggs are numerous.
Some insecticides, notably Bacillus thuringiensis, diflubenzuron (Dimilin), fenoxycarb (Insegar), pirimicarb (Aphox) are likely to be less harmful to the parasite.
These would be a better choice if they are effective against the target pest and use of an insecticide at the critical time is essential.

Biotechnological control

A powerful sex pheromone, emitted by the female midge shortly after emergence to attract males for mating has recently been identified by East Malling Research and the Natural Resources Institute. However, attempts to exploit the pheromone for control of the midge by mating disruption, attract and kill or mass trapping have not been successful to date.

Cultural control

Cultural control options are limited for apple leaf midge. Attacks tend to be more severe where trees have abundant vigorous shoot growth. Ensuring that tree growth is not excessively vigorous will prevent populations increasing.

Removal of extension growth and watershoots by summer pruning when eggs and young larvae are numerous in the shoots may reduce populations subsequently but such removal between generations may cause more intensive damage to the remaining foliage by the next generation.
Fostering high populations of predatory ground beetles by providing tussock grass in the alleyways may be beneficial.
Cultivation of the soil under the trees where the larvae form cocoons to pupate may also be beneficial though this has not been demonstrated. However, such cultivation might have adverse effects on natural enemies such as earwigs.

Monitoring using pheromone traps

The female-produced sex pheromone of apple leaf midge has been identified by East Malling Research and the Natural Resources Institute and is highly attractive to apple leaf midge males.

Sex pheromone traps are available from East Malling Research. The traps should be deployed in nurseries (or plantations where chemical control of leaf midge is to be applied) in early spring at the green cluster growth stage and monitored weekly through the season.

The height of trap deployment is critical and a standard height of deployment of 0.5 m is recommended. The lures last a season in the field. Unused lures should be stored in a freezer where they will remain viable for many years, or alternatively in a fridge where they will last 3 years or more.

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Apple leaf midge sex pheromone monitoring trap

Identification of midges in trap

Two winged fly, body about 2 mm long, abdomen dark/black in colour.
Wings sparsely clothed with short dark hairs, very few veins (only 3 are easily visible, the two anterior ones reaching the wing margin).
The first and second wing vein reach the wing margin anterior to and posterior to its apex, so that there is a lobe (see right hand diagram below).
Long legs, often broken on sticky bases, with red pools of haemolymph.
Males have a pair of claspers on rear of abdomen (Females, which are not attracted by the pheromone, have a long protractible ovipositor).
Antennae long, filiform and beaded. Conspicuous whorls of hairs on each segment. Antennae are curved back like long horns.
If other midge species caught in interesting or consistent numbers, save and photograph any interesting specimens. Do not attempt to remove them from the glue. Send to Jerry Cross at East Malling Research for identification.

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Apple leaf midge males on pheromone trap sticky base showing identification features described above

Enlarge

Apple leaf midge males on pheromone trap sticky base showing identification features described above

Monitoring experiments conducted in several orchards in the UK, New Zealand and Italy demonstrated a linear relationship between the numbers of midges captured in a pheromone trap for a particular generation and the numbers of galls that developed in the crop for that generation subsequently:

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Relationship between total catch per generation of first or second generation apple leaf midge males in standard sex pheromone traps and the number of galls formed per ha for that generation subsequently. [Best fit linear regressions on a log-log scale through the origin (dashed line) or unconstrained (solid line) are included]

The relationship indicates that each male midge caught in a trap for a particular generation corresponds to approximately 137 galls being formed per hectare subsequently for that generation, providing that there are sufficient shoots and tender young leaves present to accommodate them. Clearly the proportion of shoots and leaves galled per hectare will depend on the numbers of shoots and leaves present in the particular orchard but knowledge of these parameters should allow simple estimates to be made.

For practical purposes, a nominal threshold of 30 midges per trap per week is proposed for timing sprays of insecticide (e.g. a synthetic pyrethroid in the UK). A series of sprays at 7 day intervals is likely to be required. Good control of the first generation should result in a reduced attack in the later generations.

In most established orchards, the leaf midge is tolerated and the abundance of the pest is not monitored. However, monitoring of the timing of occurrence of eggs is beneficial for two reasons;

it indicates when application of broad-spectrum insecticides should be avoided so that establishment of the parasitic wasp Platygaster demades, a key natural enemy of the leaf midge is not impeded.
it indicates when sprays of broad-spectrum insecticides, which are active against adults, eggs and neonate larvae, should be applied for control of the pest on nursery trees.

Egg monitoring: Eggs are laid mainly on the tiny new leaves in shoot tips. Egg-laying of each generation generally occurs over a clearly defined period of a week or two, though the egg-laying period of the third generation becomes less distinct later in summer.

Monitoring of the occurrence of eggs of the apple leaf midge should ideally be done weekly, more frequently at key times.
It is sufficient to look at one representative part of an orchard per farm, choosing an early site for early warning of the egg-laying period.
The growing points of several shoots in sheltered parts of the tree should be examined for the eggs.
Often the female leaf midge can be seen in the act of oviposition.
A rough count of the number of eggs should be made.

Leaf curling damage: Each time a pest or disease assessment is made in the orchard, a note should be made of the age of the youngest leaf in the shoots which has damage.

If a larval attack is in progress, the youngest expanding leaves will show damage.
Between generations, the youngest leaves will not be damaged. This will help indicate when the next generation is likely to occur.
The occurrence of groups of damaged leaves of different ages in shoots shows a record of successive generations of larval attack.

Forecasting

Forecasting models for apple leaf midge have not been developed though it would be useful to develop temperature-based phenological forecasting models to predict the timing of occurrence of the start of egg laying in spring and the timing of successive generations.

Chemical control

None of the insecticides approved for use on apple in the UK are recommended by the manufacturer for control of apple leaf midge.

The full approval for spirotetramat (Batavia) on apples for the control of sucking insect pests offers some control of apple leaf midge, although growers may prefer to reserve its use for more difficult to control pests such as woolly aphid and 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 offer incidental control of leaf midge when applied for other pests. 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 parasitic wasp Platygaster demades (see below) 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.
Research in AHDB Horticulture Project TF 129 demonstrated that methoxyfenozide (Runner), spinosad (Tracer) and thiacloprid (Calypso) were ineffective at controlling apple leaf midge.
The synthetic pyrethroid products showed partial control in TF 129.
Deltamethrin (Decis Forte) and lambda-cyhalothrin (Hallmark, Major) are approved on apples, but in the case of lambda-cyhalothrin, there is a 365 day harvest interval, so it can only be used in nursery crops.
These synthetic pyrethroid products don’t list apple leaf midge on their product recommendations, but offer incidental control when applied to control other pests.

Application should be timed to coincide with the egg-laying period because they are likely only to be effective against adults and young larvae as they hatch from eggs. Once the larvae are enclosed in the leaf rolls, they are very difficult to control with insecticides.

Pyrethroid insecticides are contact acting only.
They are harmful to the orchard predatory mite Typhlodromus pyri and many other beneficial insects including the parasite Platygaster demades which is an important natural enemy of apple leaf midge.
For these reasons, they should only be used as a last resort and outbreaks of apple leaf midge should be tolerated.

Apple Best Practice Guide