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.