Can You Keep Spotted Wing Drosophila Out? – Exclusion Netting Trial

Shelby Jones – Agricultural Technician, OMAFRA
Pam Fisher – Berry Crop Specialist, OMAFRA
Contributor: Ellen Cole

Spotted Wing Drosophila (SWD), Drosophila suzukii, is an invasive vinegar fly that was first detected in Ontario in 2011. SWD attacks raspberries, blueberries, blackberries, day neutral strawberries and other soft fruit as they ripen, affecting shelf life and consumer appeal. SWD adults move out of infested fields and into woodlots and hedgerows in late fall. They can be found in this wild habitat until winter sets in, but very few are detected in the spring. SWD return to berry crops as the fruit begins to ripen. Growers presently manage SWD with regular applications of insecticides and tight harvesting schedules. However, this approach is costly and unsustainable in the long term. Researchers and growers continue to search for new approaches to control SWD.

One new approach may be the use of screen or netting to exclude SWD from the field. This technique has been used to keep insects from moving into greenhouse crops. Researchers in New York, Quebec and Vermont have tested the use of exclusion netting on a small scale for SWD control in berry crops. Results from these projects demonstrated that covering crops with a 1.00 x 0.60 mm ‐ 80 g/m2 net (ProtekNet) successfully excluded SWD from the area and did not affect yield or fruit quality in the short term. Lighter-weight netting (60 g/m2) was not as effective as 80 g/m2.

In summer 2015, we worked with two growers in southern Ontario to measure the effectiveness of insect exclusion netting to control SWD. The main objective was to demonstrate and gain experience with the use of this product and evaluate its potential for SWD management in berry crops.

Growers at two locations were provided with 80 gm insect netting to cover small areas of crop. They installed the netting using their own design and resources. Two rows of blackberries were covered July 4th at Site A (Figures 1 & 2) and one row of blueberries was covered July 20th at Site B (Figures 3 & 4). SWD populations were monitored inside and outside the netted area using traps baited with apple cider vinegar plus 10% ethanol.  SWD damage to fruit was assessed by collecting weekly samples of marketable fruit from inside and outside the netted area at both sites. These samples were incubated at room temperature in insect-proof, ventilated containers for three weeks. SWD flies emerging from these samples were routinely collected and counted.

Figure 1
Figure 1: Site A blackberry netting trial – outside net

Figure 2
Figure 2: Site A blackberry netting trial – inside net

Figure 3
Figure 3: Site B blueberry netting trial – outside net

Figure 4
Figure 4: Site B blueberry netting trial – inside net

At Site A, SWD was not trapped  inside the net prior to the week of August 1st. In contrast, SWD was trapped  outside the net in raspberries and in other locations on the farm for four weeks prior to this point. On August 3rd, the net over the blackberries was blown open by a storm and was closed again the next morning. In an attempt to trap out all SWD that had entered the net during this time, twelve additional traps with Suzuki bait were installed inside the net to collect SWD flies.  This attempt was not effective and SWD populations continued to increase both inside and outside the net (Table 1) after August 3rd.

At Site A, 60 fruit were collected inside (blackberries) and outside (raspberries) of the net each week for fruit assessment. SWD was not detected inside on blackberry samples collected prior to August 3rd, while it was detected outside on raspberries each week. SWD first emerged in fruit collected on and after August 4th, following the opening of the net. All blackberry samples collected after this date were positive for SWD and the number of larvae found per sample continued to increase each week.

Table 1: Site A, Blackberry SWD Trap Captures

Table-1

The blueberry plants at Site B were affected by winter injury and had a light crop. SWD was not found in traps inside the net at Site B for the duration of the trial. Increasing numbers of SWD were found in traps outside the net. Pest pressure, as measured by SWD flies in traps, was higher at other locations on the farm compared to the blueberry planting, possibly due to the light crop and the winter injury at this site. The net was taken down at the end of blueberry harvest in the week ending 15-Aug-15; 24 SWD were trapped in this area shortly after the net had been removed. When comparing the populations inside and outside the net, it can be seen that the exclusion netting was successful in excluding SWD at this site (Table 2).

At Site B, 25-50 fruit were collected inside and outside of the net each week. No SWD was detected on fruit collected inside the net prior to its removal. Fruit collected outside of the net was also free of SWD with the exception of samples collected the week of July 20th.

Table 2: Site B, Blueberry SWD Trap Captures

Table-2

This trial highlighted some of the factors for consideration and future research  if this netting is going to be cost-effective to control SWD.

Cost: The total cost of the netting is significant, and must factor in the cost of the netting fabric itself, labour to install the net, building a structure, maintenance and repairs. These costs will be offset by the returns gained from producing higher quality fruit, greater yields, and a reduced need for sprays.

Construction: Building a secure, sturdy structure for the net is critical because it must be completely sealed and free of any openings. Areas most at risk of breaches are the bottom edges of the net, the entrance and any seams in the fabric. Double doors are ideal for an entrance to allow workers to enter when needed while also providing the maximum protection against a breach.

 Accessibility: The size of the netted area and entrance to the net will determine its accessibility to workers and equipment. The structure should be large enough for workers to freely move inside when harvesting.  The door should be easy to open and close tightly.

Microclimate: Temperatures inside the net during the day were approximately 2.3ᵒC higher than temperatures outside the net. On a hot and humid day, working in the net with limited air flow can be very strenuous. Reduced ventilation and higher temperatures may pose a risk of diseases or pest development such as powdery mildew or spider mites; however this was not seen in this trial.

Other Pests: It is important to use netting in fields that are relatively free from overwintering pests such as Japanese beetle and blueberry maggot, because these pests overwinter in the field and can emerge inside the net. A control plan should be in place if these pests become a problem. Netting also excludes larger pests such as birds that can damage fruit crops, which is a substantial benefit.

Pollination: Insect exclusion netting will also exclude pollinators. Therefore it is important to install the netting after bloom but before fruit ripens. For crops which bloom and fruit at the same time, such as fall-bearing raspberries or day-neutral strawberries, bees may need to be introduced inside the structure to ensure pollination.

Weed Management: Weeds will flourish inside the sheltered warm environment of the net. Mowing could be problematic due to limited access and space inside the net. Landscape fabric or wood chip mulch  could be used to reduce weed pressure inside the structure.

Other observations: Both growers in this trial felt the demonstration was worthwhile and expressed interest in covering plots on a larger scale. Despite the breach in the net, Grower A reported a noticeable improvement in fruit quality and 150% increase in yield compared to previous years. Increased yield was a result of the extended harvesting period for blackberries and an improvement in the quality of the fruit because it was less affected by SWD. Site B had no SWD inside the net for the duration of the growing season, and bird damage was also reduced.

Exclusion netting has been tested in several other regions. See also – “Evaluation of Insect Exclusion and Mass Trapping as Cultural Controls of Spotted Wing Drosophila in Organic Blueberry Production” by L. McDermott and L. Nickerson. This article can be found in the Spring 2014 issue of New York Fruit Quarterly at http://www.nyshs.org/fq.php.

In 2015, “An Investigation of Insect Netting Trellis Systems to Manage Spotted Wing Drosophila for Vermont Blueberry Farms “ by Hannah Lee Link was published by the University of Vermont and can be found by searching for key words in the title.

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