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.
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.
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:
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.