Spotted wing drosophila
Spotted wing drosophila (SWD) is a vinegar (fruit) fly that was first reported in Britain in 2012. Unlike most other vinegar flies it can damage otherwise unblemished soft and stone fruit including strawberries, raspberries, blackberries, currants, blueberries, grapes, cherries and plums.
Latin name Drosophila suzukii
Plants affected A wide range of soft, stone and ornamental fruit
Main symptoms Collapsing fruit or fruit with small puncture holes
Caused by Larvae of a small fly the Spotted wing drosophila
Timing Damage seen in summer and autumn
What is spotted wing drosophila?
Spotted wing drosophila is similar in appearance to several species of vinegar fly in Britain, but differs in that the adult males have a distinctive spot on each wing and the adult female has a saw-like egg laying organ (ovipositer), which is used to pierce the skin of developing fruits. The female lays eggs under the surface of the skin, which turn into small white larvae (4 mm in length), which damage the fruits.
Further information on identification can be found on the AHDB Horticulture website. Unlike native vinegar flies SWD can damage nearly ripe and ripe, unblemished fruit. A native of south-east Asia, SWD is now widespread throughout much of the major fruit growing areas of the world.
SWD targets developing fruit, causing damage and resulting in deterioration. Damage from the fly is most likely to be found during warm and humid weather.
SWD can affect a wide range of fruits including raspberry, blackberry and hybrid berries, strawberries, currants, blueberries, gooseberries, grapes, cherries, plums and also peaches, figs, apricots and nectarines. It may also occasionally affect apricots, elderberries, nectarines and tomatoes. Some wild plants such as blackberries and sloes are also vulnerable. Sloe fruits persist for much of the winter and so can support SWD populations over during this time.
Fruits and berries on ornamental plants such as Sorbus can also be affected.
SWD can be difficult to control unless prompt measures are taken. Good crop hygiene and trapping can give some protection, combined with careful insecticide application and covering crops with fine (less than 0.98 mm) insect proof mesh.
Check fruiting plants frequently as soon as crops begin to ripen so action can be taken before a damaging population has developed. When choosing control options you can minimise harm to non-target animals by starting with the methods in the non-pesticide control section. If this is not sufficient to reduce the damage to acceptable levels then you may choose to use pesticides. Within this group the shorter persistence pesticides (that are usually certified for organic growing) are likely to be less damaging to non-target wildlife than those with longer persistence and/or systemic action.
Trapping and monitoring
The best way to prevent damage to fruit is to monitor for the adults and to catch them. Traps and lures are available from Agralan and some other suppliers or similar traps can be made at home. More information on trapping including demonstration videos are available from the AHDB. Grouping two or three traps together can give better results. In cherries, plums, currants, blueberries, raspberries, blackberries and other hybrid berries, it is best to put the traps on the shaded side of the plants.
In gardens or allotments that border wilder areas, copses or woods, place the traps in the fence or border line first as these will catch any SWD living in the natural habitat before they can reach the garden fruit plants. In small fruit plots using several of these traps for monitoring can help to attract adults away from fruits.
When checking traps to see if any SWD have been caught, it is can help to pour the liquid contents through a sieve over the kitchen sink. The contents of the sieve can then be gently sprayed with water using a hand held plant mister to remove the lure. Allow the insects trapped in the sieve to dry before shaking them over a white tray or card to examine them. A hand lens or magnifying glass can greatly ease the task of checking for the distinctive black spots on the wings of the male flies.
If SWD adults are found in the traps, you can exclude them from fruit by using very fine mesh netting (less than 0.98 mm), to protect the plants from the adults.
Keep trapping and monitoring until the end of November even if crops have finished to reduce populations for next season.
SWD has been found to overwinter in wild and wooded areas, so if possible such areas in the vicinity of a garden should be monitored through the winter and spring months. This will indicate if SWD is present near a garden and highlight the need for further monitoring and control measures from spring onwards.
In addition to monitoring and trapping, ensure good levels of crop hygiene are maintained in and around fruit plants to avoid attracting SWD. Pick fruit when it is ripe and don’t allow old, damaged or diseased fruits to remain on the plants or lie on the ground. Remove overripe fruits and all waste and dispose of responsibly.
Do not dispose of fruit on a compost heap as this could attract more SWD. Compost heaps can provide a perfect breeding site for SWD which will only further increase the size of the population in and around a garden. Dispose of fruit in rubble sacks, readily available from hardware stores. These need to be sealed to prevent adults from escaping and if left in the full sun for several days, the adults will be killed by the high temperatures experienced inside.
Once fruit has set covering it with netting can exclude the adult flies and reduce crop damage. These can be made at home or premade sleeves are available from Agralan.
Pesticide controlThe RHS believes that avoiding pests, diseases and weeds by good practice in cultivation methods, cultivar selection, garden hygiene and encouraging or introducing natural enemies, should be the first line of control. If chemical controls are used, they should be used only in a minimal and highly targeted manner.
In addition to trapping, garden pesticides can be used to help control the adults, although their use is unlikely to completely eliminate the fly.
The most effective means of control is to use a combination of monitoring, trapping and spraying
- Some products that can be used on a range of fruit are natural pyrethrum/pyrethrins (e.g. Bug Clear Ultra 2)
- Synthetic pyrethroid lambda-cyhalothrin (e.g. Westland Resolva Pest Killer) or deltamethrin (e.g. Provanto Ultimate Fruit & Vegetable Bug Killer) are available for use on many fruit crops
- The neonicotinoid acetamiprid (e.g. Bug Clear Ultra) is also available
Inclusion of a pesticide product does not indicate a recommendation or endorsement by RHS Gardening Advice. It is a list of products currently available to the home gardener.
Pesticides for gardeners (pdf document)
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