The use of insect exclusion mesh to protect soft and stone fruit from damage by SWD (Spotted Wing Drosophila)
Written by: Nick Buck & Michelle Fountain
SWD is a fruit fly native to south east Asia capable of causing yield reductions by ovipositing in soft and stone fruit increasing fruit susceptibility to pathogens and overall degradation. SWD overwinters in habitats neighbouring soft fruit crops especially in the winter months. These neighbouring habitats (e.g. woodland and hedgerows) provide shelter and wild hosts for SWD, which are then attracted into crops often before fruit begins to develop the following spring. Insect exclusion mesh (hereafter referred to as mesh) is one method of SWD control which acts as a protective barrier, inhibiting entry of SWD to the crop, reducing egg laying in the fruit once it ripens.
In 2019, I set up a replicated experiment with mesh door barriers from the first sign of raspberry ripening. In tunnels where the integrity of the mesh was maintained (e.g. no holes or doors left open) numbers of SWD trapped inside the meshed tunnels was significantly lower (82%) than in unmeshed tunnels. In addition, raspberry fruit produced under mesh protection had significantly fewer (94%) eggs than those without mesh.
Side effects of mesh on pests
Some studies have looked at the impacts mesh can have on other pests of soft fruit. For example, one study showed an overall 35.5% reduction in pest numbers in raspberries covered in a meshed high tunnel system compared with those unmeshed. However, there was also a 26% reduction in parasitoid wasps and predatory mites, and 32% reduction in spiders in meshed tunnels compared to unmeshed. In my 2019 study in UK raspberries, pilot data suggested that there were no effects on numbers of natural enemies or other pests, but this work will be repeated in 2021 with increased replication.
Side effects of mesh on pathogens and tunnel climate
Another gap in knowledge is the impact of mesh on crop pathogens. It is proposed that reduced air flow and increased temperature and humidity in tunnels can occur as a result of mesh installation with poor venting practices. In 2021, I will record climatic conditions including temperature, humidity, and dew point inside the meshed and unmeshed tunnels. My 2019 study suggested that raspberries produced under meshed tunnels had a significantly higher instance (12%) of sooty shoulder – an undesirable trait of raspberries. This may have been the result of increased humidity and/or less frequent flower visits by pollinating insects to remove nectar from the flowers, a factor known to cause sooty shoulder.
How and when should mesh be installed and when can I vent the tunnels?
Most commonly in polytunnels mesh is placed as a barrier at tunnel entrances – creating mesh doors, and along the outer leg rows of the tunnels. For maximum effectiveness mesh should be deployed before fruit ripening. However, SWD enter some crops much earlier, e.g. at flowering in cherry. For this reason, it might be advisable to allow pollination and then apply an effective SWD plant protection product to remove adults from the cropping area after installing the mesh. In addition, managed pollinators can be introduced to help with pollination. Ideally mesh is applied around the perimeter and across the entrance of tunnels, especially next to areas from where SWD is known to disperse. This will vary according to location and SWD pressure – which can be monitored using traps in the adjacent hedgerows and woodlands. During hot conditions when tunnels are being vented SWD is likely to be less active and does not generally lay eggs above 30oC. Venting tunnels at the hotter parts of the day are likely be less of a risk of SWD incursion because generally SWD does not prefer hot dry weather.
Mesh size requirements
Adult SWD are 2-3 mm in size, hence a mesh hole size between 0.6-1 mm x 0.6-1 mm is recommended to prevent SWD entering the crop. There are a range of mesh manufacturers and qualities so it is recommended that you speak to your agronomist about their experience with installing mesh and which they would recommend. Inadequately installed or poor-quality mesh, which tears easily creating multiple gaps significantly increases the risk of SWD entering the crop. Ideally growers designing new plantations would consider the incorporation of mesh infrastructure to make it easier to manage and vent. In addition, machinery access into tunnels and turning points on the headland could be engineered so that, where space permits, only one door into and out of the areas is required.
The price of mesh purchase, implementation and management can vary depending on manufacturer and quality, but some studies have provided estimations.
- A study assessing the benefits of anti-hail insect exclusion netting (2.4 × 4.8 mm gauge for codling moth control) in Italian apple orchards reduced the number of insecticide sprays required by 7. Growers saved up to €1,050/ha (£983.35/ha*), with costs of €2300/ha (£2155/ha*) overall, while controlling damage by multiple fruit pests and pathogens (this study also showed a 62% reduction in SWD catches in the netted treatment compared with the unnetted one, despite the mesh hole size).
- Another estimate (conducted in 2020) which included purchase of a support system, mesh, and labour costs calculated $10,000 (£7,300)/ha, for mesh that has a 7-10 year lifespan and a support system that will last much longer.
*Prices calculated through exchange rate of the year the study was conducted.
While the effectiveness of mesh in limiting SWD entry into a cropping system, and subsequent reduction in fruit damage, has been demonstrated, my PhD will consider other impacts of mesh on ecosystem services (e.g. pest and disease regulation and pollination) associated with soft fruit production. I will also calculate the costs and benefits of employing mesh in UK raspberry systems.
Mesh should be part of an Integrated Pest Management programme alongside other methods of SWD control, e.g. precision monitoring, regular fruit removal, waste treatment and rigorous monitoring throughout the year.
Written by Nick Buck (PhD student) and Michelle Fountain (NIAB EMR). The PhD studentship is supported and supervised by the University of Reading, NIAB EMR, Berry World, and Waitrose CTP. Pictures by Nick Buck.