Spotted wing drosophila

Spotted wing drosophila (SWD) is a fruit fly that was first reported in the UK in 2012. Unlike most other fruit flies it can damage otherwise unblemished soft and stone fruit including strawberries, raspberries, blackberries, currants, blueberries, grapes, cherries and plums. The RHS first saw and identified samples via RHS Gardening Advice in summer 2015.

Spotted wing drosophila HDC/The Red Brick Road Company Ltd

Quick facts

Common name Spotted wing drosophila (SWD)
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

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What is spotted wing drosophila?

Spotted wing drosophila is similar in appearance to several species of fruit/vinegar flies already found in the UK, but differs in that the adult males have a distinctive spot on each wing and the adult female has a saw-like appendage (ovipositor), 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 fruit 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.

Symptoms

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, nectarines. Other species that it may affect include 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 winter.

Fruits and berries on ornamental plants such as Sorbus can also be affected and rendered less attractive in the garden.

Control

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) mesh.

Non-chemical

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 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 wild 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 best to pour the liquid contents through a sieve over the kitchen sink. The contents of the sieve should 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 will 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 stop them from causing damage to your fruit by using very fine mesh netting (less than 0.98 mm), such as horticultural fleece or Ultra-Fine Enviromesh, 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.

Crop Hygiene

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. It is best to dispose of fruit in rubble sacks, readily available from hardware stores. These must 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.

Barriers
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

Chemical control

  • 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
  • Do not spray plants in flower due to the danger to pollinators
  • Some products that can be used on a range of fruit are natural pyrethrum/pyrethrins (e.g. Bug Clear Gun for Fruit & Veg, Defenders Bug Killer), deltamethrin (e.g. Provado Ultimate Fruit and Vegetable Bug Killer, Sprayday Greenfly Killer), lambda-cyhalothrin (e.g. Westland Resolva Bug Killer) or the neonicotinoid acetamiprid (e.g. Bug Clear Ultra). Manufacturer’s instructions on maximum dose, number of applications and harvest interval must be followed for food crops and the fruit must be listed on the label
  • Inclusion of a pesticide product does not indicate a recommendation or endorsement by the RHS. It is a list of products currently available to the home gardener

Download

Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners)

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