First reported on these islands in 2012, Spotted Wing Drosophila (SWD) poses a more severe threat than Drosophila melanogaster, the common fruit fly, because it is attracted to underripe, ripe and overripe fruit. SWD can therefore cause damage to crops both before and during harvest.

WineGB’s 2023 harvest report alarmingly tells us that SWD “was considerably worse than in previous years.” This increased activity, they suggest, is probably attributable to “the lower acidities in 2023, especially in red varieties and the increased cropping area of red varieties such as Pinot Noir Précoce (Blauer Frühburgunder) which was 30ha in 2017 and now (2022) stands at nearer to 100ha.”

Of the vineyards that responded to WineGB’s harvest survey, 49% reported they were affected by SWD last season. Some growers lost the entire crop of certain varieties while others only experienced nominal losses.

Interestingly, research in the United States has shown that hummingbirds reduce infestations by a significant percentage. But given that native hummingbird populations are not particularly active in English and Welsh vineyards, what is to be done about the problem?

The impact of SWD

Left unchecked, this fruit fly can cause the entirety of a crop to be lost. Females have a serrated ovipositor at the base of their abdomen that allows them to penetrate the skin of the fruit to deposit their eggs. That is why they don’t have to wait for grapes to become overripe like other species in the Drosophila family.

The population size can increase dramatically and quickly if uncontrolled. Like other insects, their development can be affected by temperature, but on average the flies only take a week or two to mature. A single female can lay up to 900 eggs over her life. Now that climate change appears to be bringing higher temperatures to England and Wales, the threat is apparently on the rise, borne out by anecdotal evidence.

SWD are weak fliers, but can easily be dispersed by the wind. They find a wide range of plants amenable as hosts – both wild and cultivated. This explains why their global spread has taken place with such vigour and why soft fruit crops are at such risk.

It has been suggested that red wine grapes are most susceptible, but there are plenty of reports of white varieties being attacked too. The damage they wreak is caused not just through the piercing of grape skins, leaving them open to secondary infections, but also through the action of the larvae feeding on the fruit.

Once fruit has been damaged by SWD, a clear path is available for other species of fruit fly to further attack the fruit and lay their own eggs.

What’s in a name?

Male SWD have a bold, distinctive spot along the front of each wing. These spots are visible to the naked eye but can be more easily seen under a magnifying lens. The same goes for the saw-like teeth along the distinctive ovipositor of the female flies.

Care must be taken when making an identification, however, as the wing spots take two days to fully form on the males.

SWD numbers are naturally at their highest when food is most plentiful – in the autumn. However, population levels need to be monitored year-round to get the jump on any potential infestation.

The Agriculture and Horticulture Development Board provide advice for spotting, monitoring and trapping SWD. This includes using traps at the edge of the vineyard in the early part of the season. Hedgerows and a wide range of plant species like wild blackberries are common habitats for the pest. For best results, traps should be hung a metre above ground, clear of vegetation and away from direct sunlight.

If traps in wild areas begin to show large SWD populations and the fruit is swelling, then traps can be moved to cropping areas. Fruit can also be inspected for infestation using a sugar floatation test or emergence test.

Fact file

  • Drosophila suzukii is a subgroup of the Drosophila melanogaster species group within the subgenus Sophophora.
  • Typically, SWD reproduces from April to October and overwinters as an adult. UK research has shown that the mature individuals that overwinter undergo a winter morph into a form that can withstand lower temperatures.
  • The life cycle of SWD varies according to the temperature. They perform at their optimum between 20°C and 25°C, and their activity is reduced below 10°C and above 30°C. For example, one generation can take 10 days to mature at an optimum temperature and 3 weeks or more under colder conditions.
  • Under ideal conditions, eggs can hatch into larvae in just a few hours. The larvae will pupate after about a week (this can occur within the fruit or outside of it). Pupation takes up to two weeks. After emerging, the adult flies can live for a couple of months, with females reaching sexual maturity within the first day or two.
  • Native to Asia, SWD was first reported in mainland Japan in 1916. It was first found in Europe in the autumn of 2008, where the flies were identified in the Tarragona province of Spain. The first damage to a commercial small fruit crop in Europe was recorded in Italy in 2009.
  • After the discovery of SWD in the UK in 2012, a national monitoring scheme was undertaken across 14 soft fruit and stone fruit farms in England and Scotland. There were no commercial problems reported in the initial years, perhaps thanks to cold winters and the late onset of spring.
  • SWD is not a notifiable pest. In 2012, the EU decided that because of its lifecycle and biology, official phytosanitary measures were unlikely to prevent any further spread of the population. Therefore there is no official regulation of the pest by the EU Standing Committee on Plant Health.
  • Across the world, SWD damage has been reported in a wide range of fruit crops. This includes grapes, blackcurrants, kiwis, figs, redcurrants and plums.
  • Besides crops, there are plenty of other non-cultivated plant hosts that can harbour SWD including hawthorn, holly, dogwood and elderberry.

Staying on top of SWD

In the first years when SWD arrived in the UK, control methods relied heavily on insecticides. In turn, these measures negatively impacted on well established biological controls for other pests, making chemical control problematic.

The insecticides that are currently available will only treat adult populations. Including an adjuvant in a control programme may increase its effectiveness. Products like Combi-Protec claim to attract and increase the feeding activity of adult SWD, which will improve the efficacy of any chemical control applied around the same time.

Crop health

Reducing reliance on insecticides and still keeping on top of SWD populations requires measures such as insect-excluding mesh, the sanitary disposal of waste fruit and good crop hygiene.

SWD appear to favour warm, humid conditions, so an unmanaged vineyard canopy where the leaf wall is thick and there is little air movement will help the flies to proliferate. Managing the vegetation around the perimeter of the vineyard, particularly native soft fruits like wild blackberries, may also help with reducing SWD habitats and overwintering spots.

Harvesting promptly and not leaving bunches hanging may also aid in deterring the spread of an SWD infestation.

Natural competition

Recent research has shown that SWD is not particularly competitive with other species that lay their eggs in soft fruit, so the females are put off from laying their eggs in fruit where the common fruit fly is already actively feeding. This project was led by the National Institute of Agricultural Botany (NIAB) in collaboration with researchers from the Natural Resources Institute and Berry Gardens Growers. It is hoped that this new understanding could help the advancement of future control methods, should the compound that deters SWD be identified.

Biological control

NIAB has been attempting to understand how effective native parasitoids are in controlling SWD in the wild. In non-cropped areas, it is believed that local parasitoids are contributing to 21% of the control of vinegar fly populations.

While five native species were identified, unfortunately, the pupal parasitoid Trichopria drosophilae was not one of them. This species is already commercially available in Europe for use in biological control but has not been identified in the UK, as yet, so it cannot currently be released into UK crops. This study was funded by the Worshipful Company of Fruiterers.

Elsewhere in the UK, research has also taken place into the viruses that SWD are susceptible to, as a way of identifying potential microbial biological control agents. This is a sought-after method of control as viruses can be environmentally benign and have a high level of host specificity. At the moment, the research is in a predominantly exploratory phase, identifying the viruses and assessing how well they can be isolated in the lab.

There are also studies that have been conducted to better understand the types of volatile metabolites produced by individual fruit-associated yeast isolates which attracts SWD. Since fruit naturally harbour a wide range of yeast species, this is a complex exercise, looking at how attractive various combinations of yeasts are. This work is aimed at increasing the effectiveness of bait traps in the future.

Mass trapping

Mass trapping is advocated as a control measure by NIAB. The population of SWD in crops and semi-natural habitats in the spring can be reduced through mass trapping in adjacent woodlands and hedgerows over the autumn and winter

While the flies do not lay eggs at this time of year, they may be active and feeding on warmer days. The absence of competing fruit makes traps more effective, minimising populations at the start of the growing season.

Sterile insect technique

One newly established technological approach to reducing SWD populations is the Sterile Insect Technique. Sterile males are released at repeated intervals throughout the season. They outcompete the wild males, leaving the females infertile. By keeping populations low, reliance on chemical insecticides is also reduced.

Early trials of these sustainable insect control solutions on strawberry in open polytunnels have been described as “encouraging”. SWD levels remained low compared to similar crops treated with agrochemical products, which mainly only impact adult flies.

Reducing insecticide usage

NIAB is working with Russell IPM and Microbiotech to develop control strategies that combine lower doses of insecticide with bait that attracts SWD.

“Results show that using this strategy can reduce the amount of insecticide applied to an area in a single growing season by up to 95% resulting in less environmental contamination and lower residues in fresh produce,” the NIAB reports.

Reflections

From Winding Wood Vineyard 2023

In late September, we noticed a plague of fruit flies had invaded the Pinot field. It was some years ago when we had a similar outbreak.

Vinegar traps were put out to catch them but the damage to some of the bunches had been done.  These flies are called Spotted Winged Drosophila (Suzuki fly) of which the female lays eggs under the skin of the berry which then hatch. To the eye, the berry exhibits a brownish colour, is squishy to the touch, and has an exit point. This leads to sourness.

As a result, we laboriously spent days on our hands and knees, extracting each damaged berry. The effect on the bunch if not picked out can be ruinous to the crop. We hear from the winemaker that many vineyards are infested. Why? Maybe a perfect storm with the warm and wet conditions experienced over the summer.

Taken with permission from the Winding Wood Vineyard blog on www.windingwoodvineyard.co.uk

Chardonnay in the press

Collaborative working in the UK

The UK Plant Health Service is working in collaboration with industry to monitor SWD in the UK. There is an SWD Working Group which aims to track the spread of the pest and provide advice on control and management.

The SWD Working Group has produced posters for growers and packhouses, which detail identification and the floatation test. Find them through www.agriculture.ahdb.org.uk. The group are also working with the Chemical Regulation Directorate to identify and approve potential new crop protection products which may contain and control the pest.

The AHDB has invested more than £1.6m, over a decade, into SWD research. This includes funding for research projects with NIAB, co-funded by DEFRA, British Summer Fruits, Innovate UK and The Worshipful Company of Fruiterers.

The initial NIAB project tested traps, bait and monitoring devices to assess their effectiveness and give growers practical solutions for monitoring SWD populations on their land. This has given an increased understanding of how adult SWD live and overwinter. The research confirmed the importance of removing damaged and diseased fruit and disposing of them through fermentation before reincorporating into the soil.

NIAB’s work has also experimented with agrochemical control agents to test them in UK conditions. The outcomes of these research projects have led to the assembly of a set of grower guidelines which can be found on an AHDB factsheet.

Read more: horticulture.ahdb.org.uk/knowledge-library/management-and-control-of-spotted-wing-drosophila

Find out more about the Working Group: www.archive.ahdb.org.uk/knowledge-library/the-spotted-wing-drosophila-working-group

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