Spotted Wing Drosophila (SWD) is an invasive fruit fly first reported in the UK in 2012. It was identified at the NIAB’s East Malling site in Kent and has been a thorn in vineyard managers’ sides ever since. This winged pest hails from Japan and has gradually spread worldwide, targeting soft fruit crops.
Unlike Drosophila melanogaster, the common fruit fly found in the UK, Drosophila suzukii is attracted to underripe fruits as well as the ripe and overripe, so it can attack crops both before and during harvest. It has been found across habitats and in a wide range of plant species, including wild blackberries, which helps to explain why it has spread so easily.
If uncontrolled, this pesky fly can cause entire crops to be lost. Aware of the dangers, scientists had already been mapping SWD’s spread into mainline Europe. The UK fruit industry formed a pre-emptive SWD working group in 2011 to consider how the threat could be managed when it inevitably arrived.
The Agriculture and Horticulture Development Board has spent more than £1.6m over the last decade on research projects to monitor SWD, examine its habitats, look at effective crop management strategies and develop controls.
Getting to know Spotted Wing Drosophila
Although different species of Drosophila larvae cannot be identified on sight, the adults have characteristics that can be seen with the naked eye or with a magnifying glass. It is worth keeping an eye open for SWD when walking the vineyard, paying particular attention to the fruit where they land to mate or lay eggs.
Males have a large spot along the front of each wing, which is bold and distinctive. Females can be recognised by their unusual serrated ovipositor, which may need to be identified through the use of a lens. Located at the base of the abdomen, the saw-like teeth on this appendage allow them to penetrate the skin of the fruit to deposit their eggs. Other species of the Drosophila family do not have this and must lay their eggs in overripe or rotten fruit, where the skin has already been breached.
Red wine grapes are most often attacked when veraison begins, but white grape crops are at risk too, when their sugar levels are high enough. As well as damaging the fruit through piercing the skin and the action of the larvae feeding, SWD attack increases the risk of secondary infections like botrytis and sour rot.
Population size can increase rapidly. Depending on the temperature, SWD takes a week or two to mature into adulthood, and a single SWD female can lay up to 900 eggs in her adult life. For this reason early identification and swift action are paramount. This occasional pest may become more of a feature in our vineyards as the climate slowly warms.
Spotting the Spotted Wings
SWD numbers will naturally be at their highest in the autumn when their food is at its most plentiful. However, there is a case for monitoring populations year-round. The attractants used will be more effective when there is less food naturally available, highlighting a potential problem in the making for next season.
Professional traps with purpose-built attractants are available and highly effective. The AHDB currently recommends products like Dros’Attract from Biobest. At a pinch, something as simple as jam or cider vinegar can be used, but this will be more time-consuming as the trapped insects will need to be identified.
The AHDB offer practical advice for monitoring and trapping SWD:
- In the early part of the season use traps at the edge of the vineyard; in hedgerows for example.
- Hang traps a metre above the ground, clear of tall vegetation and out of direct sunlight.
- Once traps in wild areas catch large populations and the fruit is swelling, commence monitoring in crops.
- Samples can be sent to the entomology team at NIAB EMR for confirmation if you are unsure of the identification.
- Fruit can be inspected for signs of infestation using a sugar flotation test or emergence test.
Dealing with SWD
Early treatment is key to keeping an SWD infestation under control. Chemical options are available, but only effective on adult populations. Your integrated pest management strategy can incorporate other approaches that do not require chemical intervention.
Vineyard hygiene is extremely important to prevent the spread of SWD. Damaged and fallen fruits should be removed from the area. Infected grapes should not be discarded into compost heaps as the flies will be able to feed and reproduce there. It’s worth knowing too that the females can overwinter.
The current guidance is to completely enclose waste fruit for at least 48 hours at 14°C, longer at lower temperatures, to kill the larvae. Even after this, care needs to be given to how that waste is finally incorporated into the soil as it will still be attractive to adult flies.
Where possible, crops should be harvested promptly to minimise the risk of secondary infections like botrytis after an SWD population is identified. The longer a crop is left to ripen, the bigger the risk you take.
Higher temperatures contribute to the vigorous spread of SWD. As they favour warm, humid conditions good canopy management that encourages air flow can reduce the SWD risk as well as aiding ripening.
Kaolin, a white clay mineral, can be applied to the fruit to deter the insects from laying their eggs there. Kaolin is inert and toxicologically harmless as well as having no effect on the later vinification of the grapes. However, this would be a resource-heavy approach. A good coating is needed and it can be a partial barrier to sunlight too, potentially delaying ripening.
An insect exclusion mesh is a practical solution for crops grown in polytunnels but is less pragmatic in a commercial vineyard. A simpler approach may be to manage wild host plants around the vines, trimming back blackberries for example.
Research continues to find a way to deal decisively with SWD. A NIAB led research project recently found that the presence of common fruit fly larvae in laboratory media deterred SWD from laying eggs. Work is now underway to identify the compound causing this effect to use in future IPM strategies.
Hope is also being placed in the Sterile Insect Technique, developed by BigSis in collaboration with NIAB, which has been offered as a commercial service to fruit growers for the first time this season. Sterile males are introduced to the crop to mate with wild females, who therefore fail to produce viable eggs. Trials on strawberries in open polytunnels have been encouraging, with SWD levels remaining much lower than in comparative sites treated with chemical products.
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