Spotted Wing Drosophila

Drosophila suzukii (SWD) is no longer a fringe pest for UK grape growers it is now established, active, and poised to exploit late season ripening fruit. Native to Southeast Asia and first detected in Europe in 2008, SWD’s rapid spread is driven by its unusual biology: unlike the common fruit fly (D. melanogaster), SWD females possess a heavily sclerotised, serrated ovipositor. This enables them to cut into the skin of sound, unblemished berries, particularly red varieties and clones with thinner epidermis, and deposit eggs directly into the pulp.

Once inside, eggs hatch within 1–3 days (temperature dependent), and larvae feed on the pulp, leading to softening, collapse, and secondary infection from Botrytis, sour rot bacteria, and acetic acid yeasts. Yield loss is compounded by downgraded fruit quality, with contamination thresholds in wineries often as low as 1–2% damaged berries.

Seasonal Risk Outlook

The forecast for a warm, wet late summer in 2025 creates optimal conditions for population expansion. SWD thrives in temperatures between 20–25 °C and high relative humidity (>70%), with overlapping generations possible every 10–12 days. Populations can increase exponentially during September and October, exactly when UK grapes are at peak ripeness. This year, risk is high, and late-ripening reds are particularly vulnerable.

Identification – Male vs Female

Correct identification is vital for timely intervention.

Males – Smaller and easier to confirm visually; a distinctive single dark spot near the tip of each wing. Spots are visible under a hand lens or good lighting.
Females – Slightly larger, with no wing spots. Identification requires recognising the serrated ovipositor under magnification (20x). These are the damaging individuals, often overlooked until fruit damage is obvious.

Figure 1. SWD Male vs. Female

Photo Credits: Sheila Fitzpatrick, Agriculture & Agri-Food Canada, Pacific Agri-Food Research Center, Agassiz

Key point: The presence of males in traps confirms females are active and laying – by the time you see males, the pest cycle is already underway.

Monitoring

Timing – Start trap deployment at veraison, especially once berries approach 50 °Oe, when skins soften enough for oviposition.
Trap Design – Use baited traps with yeast-sugar solution (e.g., 7g baker’s yeast + 2g sugar in 100ml water) or proprietary lures.
Placement – Shaded, humid areas within the fruit zone, 1–1.5 m above ground. Space traps evenly across the block, with higher density near woodland edges and sheltered areas.
Frequency – Check traps at least twice weekly; increase to every 2–3 days in peak risk periods.
Data Use – Keep records of catches and temperature/humidity to help predict peaks and refine intervention timing.

Cultural Controls

Sanitation – Remove and destroy fallen, split, or damaged fruit daily. This prevents larvae completing their development and re-infesting the block.
Canopy Management – Open canopies reduce humidity and make conditions less favourable for egg-laying. While some literature suggests 35–45% leaf removal in the fruit zone, experience shows up to 80% removal (without impacting ripening) significantly reduces SWD attraction.
Host Removal – Eliminate nearby wild or unmanaged host plants, particularly Rubus fruticosus (brambles), Sambucus nigra (elder), Prunus avium (wild cherry), and late-fruiting ornamentals. These act as breeding reservoirs between crops.

Chemical Controls

When monitoring confirms risk, chemical control may be necessary as part of an Integrated Pest Management (IPM) approach.

Tracer (spinosad) – Broad efficacy against adults; translaminar activity; 14-day HI. Avoid consecutive applications to delay resistance.
Hallmark (lambda-cyhalothrin) – Fast knockdown; shorter persistence; 7-day HI. Best used strategically, not repeatedly.

Rotation between chemical groups is essential. Overuse of one mode of action accelerates resistance development – a documented issue in high-pressure regions of mainland Europe.

References:

UK Government SWD guidance
DEFRA pest risk analysis for SWD

https://swd.russell.wisc.edu/biology/

Integrated pest management (IPM) for spotted wing drosophila (SWD) control | AHDB

Why use Spotted Wing Drosophila (SWD) monitoring traps? – Andermatt UK