Spotted wing drosophila (SWD) - spotting, monitoring and trapping

Evidence from other countries has shown that early detection and rapid response is crucial to minimising the impact of SWD on soft and stone fruit crops.

Priority is to begin monitoring early to ensure that control measures can be put in place as soon as adults are found and before they have time to start laying eggs on developing fruits. The recommendation is for monitoring to start in March and continue through to November.

To ensure that damaged fruit is not sent to market, it is vital that fruit is inspected and sampled both before picking and afterwards to check for the presence of larvae. Delivering affected fruit will damage a grower’s reputation with the customer.

Monitoring for adult SWD

A number of monitoring traps have been tested in the UK and this is likely to continue as newer, improved traps are developed. Experience to date suggests that the most practical trap which is easy to use and effective at catching spotted wing drosophila is based on the Droso Trap.

Research continues to search for the optimum baits for our conditions and crop types and this site will be up-dated as new information becomes available.

At the start of the production season, it is best to use traps in wild areas of the farm, such as hedgerows and woodland. Traps in wild areas should be hung 1 metre above ground level and out of direct sunlight. Clear any tall vegetation away from the trap so that it’s easily accessible. As soon as high populations are being caught in wild areas or in perimeter traps, then start monitoring in crops, but not until fruits start to swell.

Use two traps per cropping situation about 10 metres from the perimeter of the crop, but remember that traps should be hung on the shady side of crop rows. Precision monitoring traps can also by placed around the perimeter of the crop in adjacent wild areas at a 2 metre spacing.

• In stone fruit, cane fruit and bush fruit crops, hang the traps at one third of the canopy height

• In outdoor strawberry crops, position above the ground but below the spray boom height

• In tunnel strawberry crops, hang in the leg row at a height of 10 cm

• In table-top tunnel grown strawberry crops, hang from the table-top, but positions as close to the hanging fruit as possible

If you are unsure whether or not you have caught SWD, samples of the adults can be sent to the entomology team at NIAB EMR for confirmation (English Growers) or to Gaynor Malloch at the James Hutton Institute (Scottish Growers). All such samples will be treated in confidence by both EMR and the James Hutton Institute.

If submitting samples caught in the liquid bait, the adults can be removed by pouring the liquid through a sieve over a sink. The adults should then be placed in a small sample tube with some liquid bait, which can be sent through the post in a Jiffy bag.

It is important to monitor for damaged and infested fruit both pre and post-harvest. Detecting a problem before picking can save the labour required to pick infested fruit, allowing control measures to be applied before the next pick. Detecting a problem after picking can save on packing costs and prevent damaged/infested fruit reaching the market.

Sampling fruit pre-picking

• Sample weekly from all plantations at risk
• Select more than 100 ripe fruits from a transect across the crop
• Include fruits from the edges of plantation near wild hosts
• Collect fruits from the lower centres of plants and avoid fruit on the ground

Sampling fruit post-picking

• Decide upon the punnet sampling frequency (eg. 1/100)
• Take a sample from each crop at each picking time
• Record numbers: lot size, sample size, number where pest detected
• Work out incidence and confidence intervals
• Reject the consignment if SWD larvae are found

How to inspect fruit

Whether sampling pre-picking or post-picking, the fruit should be inspected for signs of infestation on the surface and inside the fruits. Several techniques to assess if fruits contain larvae have been assessed in the UK industry funded research project (SF 145). Results demonstrate that sugar floatation is the most sensitive and rapid test for growers to use.

• Make up a sugar solution (170 g sugar per litre of water – 15 oBrix)
• Gently crush the sample of fruit in a clear plastic bag and add the sugar solution to the fruit
• Stir the fruit, let it settle for 10 minutes, stir again, and assess 10 minutes later
• Any living larvae will float out of the fruit

An emergence test should be undertaken alongside a floatation test.