Management of the Spotted Wing Drosophila using High Tunnels

PROJECT BACKGROUND

Scenic Valley Farms is a family owned farm that designs and manages high tunnels, computerized climate control systems, and subterranean solar thermal heating systems. The company currently manages fifteen climate controlled high tunnels that produce organically certified tomatoes, peppers, cucumbers, blackberries, strawberries, herbs, ginger, turmeric, and garlic. The farm is 27 acres in size but only roughly four of those acres are used for production. We market and distribute the produce to dozens of grocer co-ops, small supermarkets, wholesale dealers, and restaurants in Viroqua, La Crosse, Chicago, and the Twin Cities metro area.

In order to boost organic matter and promote healthy soils we apply organic compost and rock powders to the fields underneath our high tunnels. We’ve been using these practices since the farm started operations seven years ago. We also use innovative solar thermal heating methods that warm the soil and air while reducing on farm greenhouse gas emissions. We now have three high tunnels that utilize solar energy and have found that our propane fuel usage has dropped as a result. Our solar thermal high tunnels take a 4 ½ month field grown season and extend it to 9 months. We are able to grow two main crops such as tomatoes and strawberries, tomatoes and cucumbers, tomatoes and garlic. Growing two crops adds 50% to the high tunnel’s revenue. We are able to grow tropical plants such as ginger and turmeric to maturity which is 9 months. We are able to grow and harvest strawberries from May through November in our vertical growing system. Our vertical strawberry growing system can accommodate up to 18,000 plants per 30 x 96’ tunnel producing ½ to 1 pound of strawberries per plant. An equivalent field grown footprint

Contains 1800 plants. All of our produce is certified by the Midwest Organic Services Association. We also reduce soil erosion by incorporating natural and synthetic mulches.

PROJECT DESCRIPTION

GOALS

Our proposed solution was to develop procedures that attempted to prevent as many SWD as possible from entering the high tunnels (Objective 1) and effectively control them using organic methods once they have entered the growing structures (Objective 2). In order to prevent population build up, we installed fine mesh insect screen across the side walls and any other openings of the tunnel. This solution prevented sizeable numbers of the pest from entering the high tunnels. The remaining population, which was much easier to manage due to its low numbers, was managed through the application of organically approved insecticides. Care was taken to ensure that the pollinators within the tunnels were returned to their hives before any applications were delivered.

PROCESS

The work related to the grant funds mainly related to three activities: (1) monitoring the presence of the Spotted Wing Drosophila (SWD), (2) installing insect netting on the high tunnel greenhouses as well as placing the same type of netting on field grown strawberry crops, (3) and applying organically approved insecticides once the SWD were detected.

The unusually harsh winter of 2013 – 2014 disrupted our research activities. The varieties of blackberry canes grown in the high tunnels of Scenic Valley Farms (SVF) and Prairie Belle Enterprises must be overwintered in order to allow the primocanes to produce fruit the following year as floricanes. The varieties (Natchez, Ouchita, Apache, et.al) were developed by a breeder in Arkansas and sustain damage to their canes if temperatures drop below -12° C. We had installed 170,000 BTU thermostatically controlled tube heaters in the blackberry high tunnels but due to numerous nights that dropped below -20°C we had problems maintaining pressure in the LP lines and the heaters malfunctioned. As a result the canes sustained substantial damage.

Prairie Belle Enterprises made the decision in early May to remove the floricanes and allow the primocanes to take advantage of more exposure to sunlight and more access to soil nutrients. Connie King made this decision reluctantly, but felt she had no choice due to the brittle nature of the floricanes and the almost complete lack of flower bud development.

SVF also noticed severe damage to the floricanes and minimal flower bud development at about the same time. We decided to remove approximately 90 percent of all the floricanes with the hope that the remaining 10 percent might produce enough blackberries to conduct research on and for our own personal consumption.

At both farms, we spent a considerable amount of time installing the insect netting on the high tunnels. What we anticipated would be a fairly straightforward, simple job turned out to be anything but that. The problem is that high tunnels are not designed to prevent extremely small insects like the SWD from entering. On the blackberry high tunnels, we also removed the plastic from the end walls to allow for better airflow during the warm summer months. So we needed to design a system that would be both effective as a blockade against the SWD and versatile enough to be used in subsequent years.

We elected to use insect netting manufactured by ProtekNet,

http://www.duboisag.com/en/proteknet-exclusion-insect-netting.html . The dimensions of each roll of netting is 6.5’ X 328’. The mesh is 1.00 mm X .60 mm. This size opening was chosen because it was small enough to prevent SWD from entering but large enough to only restrict approximately 20 percent of natural airflow.

We secured the insect netting to the side walls with a combination of uchannel/wiggle wire and wood lathe. We first removed the wiggle wire from the uchannel on the hipboard that held up the plastic on the roll up and down sidewalls. We then attached the insect netting and reattached the sidewall plastic with the same lengths of wiggle wire. Using self-tapping screws, we attached uchannel to the corner bows on the sidewalls and then secured the insect netting to these lengths of uchannel. On the sidewall baseboard, we pulled the insect netting tight, and then used four foot lengths of wood lathe and .5” wood screws to secure the netting to the baseboard. This method sealed up the sidewalls tightly.

The end walls and especially the doors presented more of a challenge. We removed the poly from the end walls and using two lengths of insect netting, secured it using the existing wiggle wire. We then reattached the end wall plastic over the netting using a second set of wiggle wire in the same uchannel. Along the ground, openings were closed using ground stakes spaced roughly one foot apart. A similar method was used on the doors. To block openings along the door headers and jams, we folded the netting and stapled the edge to the door frame to create a “seal”. This didn’t completely seal up the doors but all along we knew that it was impossible to prevent all SWD from entering the tunnels.

Sealing the tunnels in this manner prevented pollinators from entering the tunnels but fortunately both farms use bumblebee hives within the structures. There is enough water occurring naturally in the growing plants in the tunnel to supply the bumblebees.

We also installed a 6000 CFM thermostatically controlled exhaust fan in two strawberry tunnels to increase airflow since the netting would cause temperatures to be higher than normal during the summer months. We also covered the tunnel with 30% shade cloth to cool the tunnel during the heat of the summer.

At SVF, we enclosed three tunnels using the methods described above: two blackberry tunnels, and a tunnel of day neutral strawberries. Our four tunnels of June bearing strawberries need no netting because we could harvest all the berries by the middle of June prior to the SWD arrival. At Connie Kings’ farm, a single blackberry tunnel was enclosed. At Jesse Down’s farm a single high tunnel of peaches was enclosed.

In each tunnel we placed two SWD traps evenly spaced throughout the growing area. The traps consisted of plastic containers with sealable covers and small perforations to allow the flies to enter. Half of the traps contained a vinegar mixture and the other traps were a mixture of sugar dissolved in water. In each trap, we placed a drop of dish soap to prevent the flies from leaving the traps. 

We also placed traps outside to compare the SWD population with those present inside the tunnels. We placed two traps outside on either end of the high tunnel. The traps were placed along the edge of the woods bordering the fields and next to a row of strawberries planted outside (described below).

Although not described in the original grant proposal, we also conducted research on growing strawberries in an outdoor field environment. In early May, we planted day neutral strawberries in a 100-foot-long raised bed. The berries were then covered later in the summer using row cover hoops and insect netting and the netting was attached to the hoops using cloth pins. We also were in contact with Bill McNitt of McNitt Farms in Illinois who developed a method to control SWD in outside grown thornless blackberries. See below for these results.

PEOPLE

• Erik Gundacker is the owner of Scenic Valley Farms. He designed the netting system to protect the high tunnels and sourced the materials. He also oversaw the installation of the netting. During the course of the project, he analyzed the data coming in about SWD population levels and developed strategies for responding to this data. He also presented the research findings to the public through several conferences.
• Jesse Downs is the co-owner of Down Home Farms which is located in Reads town, WI. Jesse’s farm grows garlic, onions, squash, strawberries, raspberries, and various other certified organic produce. Jesse has also pioneered the production of high tunnel grown organic peaches in the region, using the technology to successfully overwinter the crop. Jesse performed much of the installation of the insect netting, circulation fans, and traps. He also was primarily responsible for monitoring the traps for SWD and recording the approximate levels of the insect within each trap.
• Connie King is the co-owner of Prairie Belle Enterprise which is located in Dewitt, IA. They produce organically certified blackberries, tomatoes, peppers, and other fruits and vegetables in high tunnels. Prairie Belle Enterprise markets in local farmers markets and restaurants. Connie King conducted a separate experiment at her farm that essentially mirrored the practices of SVF related to protecting blackberries from SWD. She installed the insect netting, collected data on the installed traps, and reported this data to SVF.

RESULTS

Growing Seasons: 2014

As expected, the remaining floricanes in the SVF high tunnel during the 2014 growing season produced minimal fruit. The traps within this tunnel showed no presence of the SWD until late-July and the population was very small with just a few insects in the trap nearest the fruiting canes. At this point, we collected the bumblebees and removed the hive in the evening. We then applied Entrust SC Naturalyte at the rate specified in the label and returned the hive to the tunnel the next morning. The berries only continued to fruit for another week and showed no signs of SWD infestation.

The traps placed in the tunnels of June-bearing strawberries showed no presence of the SWD. But this was hardly surprising considering that we finished harvesting that crop in early June and the SWD does not typically begin to appear in southwest Wisconsin until the middle of July at the earliest. The tunnel that had the highest level of infestation was the tunnel planted in day neutral strawberries. This is primarily because day neutral varieties bear fruit throughout the summer and fall months, and the SWD population peaks in August. For several days in August, the doors to the day neutral tunnel were accidentally left open. Shortly afterwards, we began to notice larvae in the fruit and increased counts of both male and female SWD in the traps. At that point, we expected to have a major challenge on our hands to control the population and prevent significant damage to the fruit. This did not turn out to be the case however. We applied Pyganic Crop Protection EC 5.0 II for three consecutive days because it is a contact insecticide.

We then applied Entrust, waited a week, and then applied Entrust again. The most challenging part of this treatment program was making sure that we removed the bumblebee hive from within the tunnel and then reintroduced the hive after a safe interval. After two weeks, signs of adult SWD in the traps all but disappeared and the fruit showed minimal damage. During this period, and for the remainder of the growing season, we were careful to instruct the harvesters to remove any overripe berries on the plant or berries lying on the ground. These berries were then buried at a depth of approximately two feet.

In our opinion, the primary reason we were able to control the SWD outbreak in this tunnel is because of the insect netting. It largely prevented new adults from entering the tunnel while we were attempting to control the population already in the tunnel. Without this protective barrier, we would have been forced to apply insecticide throughout the entire harvest season which would have resulted in much higher labor and material costs and a less healthy crop of berries.

During the period of infestation, we were forced to cull approximately 30 – 35 percent of the harvest but these figures dropped back to their more typical 10 – 15 percent range once the infestation was brought under control.

The day neutral strawberries growing in the outdoor field environment were uncovered until late summer and as a result the fruit became heavily infested with SWD larvae and was completely unmarketable. Once the hoops and netting were in place, we applied Pyganic Crop Protection EC 5.0 II directly through the netting for three consecutive days and then switched to a single application of Entrust. We followed this six days later with another application of Entrust. By the third week there was almost no indication of SWD and we were able to continue marketing the fruit. In order to harvest the berries, we removed the netting and then reattached it afterwards with the clothespins. The key again seemed to be the insect netting.

All the traps outside indicated the presence of both male and female adult SWD. The insects were first recorded in the traps in late July. The number of SWD in the traps peaked in mid-August and they largely disappeared by early September. As anticipated, the trap with the highest recorded number of adult SWD was located near a stand of wild blackberries. Growers attempting to control the SWD population on their farms might thus consider removing wild stands of berries located near their fields. Last growing season was unusually cold and wet throughout May and June, and we expect this might have delayed the onset of the SWD. In previous years, signs of the SWD began to appear in early July.

Growing Season: 2015

The winter was much less harsh and as a result most the blackberry canes survived. There was minor damage to the insect netting over the winter but we were able to repair it relatively easily. The traps did not begin to show SWD present until around mid- to late July. At that point, we began the same treatment procedures as those described above. The population levels within the high tunnel were significantly reduced and we were able to cut back on the spraying regimen to reduce labor costs and the amount of pesticide applied to the plants. For reasons we still have not determined, however, the yields on the blackberries were roughly half what we expected.

There is the possibility that the 2013-2014 winter might have damaged the crowns of the plants and the canes will still require more time to fully recover.

We had more success in controlling the SWD in our day neutral strawberry high tunnel. The fruit flies were present in the tunnel throughout the year but by regularly checking for breaches in the netting and keeping the doors closed, we were able to maintain their population at a level that did not significantly reduce the overall yields. Throughout the season we typically had to cull 10 – 15 percent of the berries due to larvae infestation, as well as other problems. We were satisfied with this cull rate, however.

The outdoor, field grown strawberries were protected again using row hoops and netting. The main problem we faced, however, was in allowing pollinators to reach the plants. We were forced to remove the netting during the pollination periods, but this did not result in SWD infestation because the insects had yet to appear in the region. We also faced problems with airflow to the plants. This summer was considerably warmer, and the netting restricts roughly 20 percent of air flow, so yields were likely reduced because the plants suffered from heat stress.

During the summer we had to remove the netting for short intervals of time to let the pollinators in and to pick the berries. The SWD, when they were present, infested the patch. We sprayed using Pyganic through the netting and did not have to remove the netting. This method of control was somewhat successful.

At McNittt’s Farm in Illinois, Bill claims he controlled the SWD by picking all fallen blackberries, picked berries before they became over ripe. When SWD were present he used the following sprays at a three day interval: Entrust and Pyganic on a 3 day rotation and every 6 days added Mustang Max (Non OMRI).

At Down Home Farm, Jesse had no SWD in his peach tunnels though he had SWD in traps outside the tunnels. The insect netting controlled the SWD.

Once again, the outdoor traps contained SWD. The insects began appearing earlier this year than the last, mostly due to the warmer weather experienced. The number of insects peaked in early August and then began declining thereafter. The traps near the wild blackberries also showed the highest number of SWD, similar to the year earlier.

Growing Season: 2016

In the winter of 2015/2016 SVF maintained a minimum temp of -12° C/10° F in both its Rosemount and Readstown blackberry high tunnels. In late March it appears we have had over 95% winter survival rate. We used 100 gallons of LP at $1.20/gallon to heat a 30’ x 96’ high tunnel and 44 gallons of LP to heat the 30’ x 64’ high tunnel. We had two 20” circulation fans located in the ceiling running when the heater was active.

DISCUSSION

One of the main lessons we learned was the importance of good harvesting practices. If we removed overripe berries and picked up berries that had fallen on the ground, we were able to deny the SWD their favorite habitat for reproducing: ripe and overripe fruit.

We also learned the importance of carefully monitoring the traps. We placed two traps within each tunnel and two traps were placed outside near each tunnel. If we detected SWD inside the tunnels, we immediately would pick all ripe berries, remove the hives, and then spray two consecutive nights with Pyganic, followed by a third night with Entrust. Most of the time, this procedure reduced SWD populations to acceptable levels.

If the SWD population becomes significant enough, no amount of netting, spraying, or good harvest practices can reduce the population. So it is very important to remain diligent in the fight against these pests.

We also noticed the SWD will breed within culled tomatoes because usually they are overripe. So we developed a practice of burying culled tomatoes after each harvest to prevent the insect from breeding in these piles.

The netting we installed will also work as a deterrent against the cucumber beetle. It is important to install the netting before the cucumber beetle appears. The grower must take into account though that the netting will reduce airflow, so it is advisable to install at least two circulation fans within the tunnel.

Another important lesson is the role that wild berry patches play in sustaining SWD populations. As previously noted, we discovered much higher amounts of SWD in the traps near a wild blackberry patch. So it might be worthwhile for growers to remove these stands if they are able to.

OUTREACH

• We hosted a field day in September for interested members of the community to come and visit our farm. At these field days we provided information to visitors about the different practices we were using to control the SWD and the results of the practices up to that point and time
• We gave project status presentations to the Minnesota Fruit and Vegetable Growers Association, and the Minnesota High Tunnel Growers conference
• We also presented at the 2016 NCR-SARE Farmer’s Forum.