Insect netting for fall raspberries and late blueberries

Jerry Smith and Lisa Holderness operate Deer Ridge Farm in Guilford, Vermont. This diversified, small-scale organic farm produces cut flowers, maple syrup, small fruits and vegetables on a handful of hillside acres. The first confirmed report of spotted wing drosophila in Vermont was at this farm in early September 2011. Since then, the farm has had very high SWD pressure. In 2013, we set up a trial to explore the effectiveness of insect netting in excluding SWD from fall raspberries.The farm has about 0.25 acre of raspberries, primarily fall-bearing, and 60 mature blueberry plants. SWD was found in both these crops around the first of August in 2012 and 2013. Unprotected fall raspberry fruit was almost completely infested and unmarketable until about the first of October both years. After that date, SWD infestation declined greatly, and most raspberries picked in October were marketable. Unprotected blueberries were picked once and then abandoned, as all fruit was infested.

         Photos by Vern Grubinger.


Photos by Vern Grubinger.

In 2013, a small trial was conducted with ProtekNet 25 insect netting to exclude SWD. The 21-foot-wide netting was placed over a 30-foot-long section of a row of Heritage raspberries, over a 75-foot-long section of Caroline raspberries and over a 50-foot-long section of Bluecrop and Berkeley blueberries. To support the netting over the crops, two 10-foot-long sections of electrical conduit were joined together to make 20-foot metal hoops. The ends of the conduit were forced about a foot into the ground, so the 21-foot-wide net covered the hoops with about a foot of net remaining on each side. The edge of the net was rolled up over wooden 2x4s to hold it down. The net overlapped at the tunnel ends, where it was secured with bricks on the ground; these were lifted when entering the net to pick berries.

Netting was applied over Heritage on July 1 because the primocanes from the year before had not been removed and were being fruited in the summer. The net remained in place all season. There was almost no SWD damage in both the summer crop and the fall crop borne on new primocanes. However, pollination appears to have been reduced, as berry quality was lower than normal, with smaller, somewhat crumblier berries being produced. There was no harvestable fruit from the uncovered Heritage until October, when it was no longer infested, and in that month the uncovered plants yielded higher-quality fruits that were about 50 percent larger than those under the netting.

Netting was applied over Caroline on September 1. By then the early fruit was already infested with SWD. After netting was put up, all fruit was picked and removed from inside the netted area. Three red plastic cup traps baited with a flour/sugar/yeast mixture plus a smaller cup with alcohol drowning solution were placed inside and outside the net (aka two-cup traps). The traps were emptied and the bait changed weekly. Ten days after being covered with netting, the Caroline fruit was virtually free of SWD, and it stayed that way until the end of the season.

Toward the end of September, Smith observed a lot of male and female SWD congregating inside the netting on the south side, but they did not infest the fruit. Clean fruit was picked until October 20, and the quality was terrific, with no reduction in size compared to the uncovered Caroline plants. “It may be better to lose a few weeks of early production by delaying netting if this allows pollination to take place so the rest of the season’s crop is of high quality,” Smith noted.

Netting was applied over the blueberries on July 1. There was no sign of SWD infestation in the covered fruit, which was picked all season. Blueberries outside the netting were picked once and then abandoned due to near-complete infestation with SWD.

        Graph by Vern Grubinger.

Graph by Vern Grubinger.

In addition to the ProtekNet 25 (netting that weighs 25 grams per square meter), there is a more durable netting called ProtekNet 80 (80 grams per square meter) in different widths. Whatever type of netting you buy, be sure that the mesh opening is 1 millimeter square or smaller to exclude SWD.

Insect netting is not inexpensive, but it should last for a number of years if handled carefully. The annual cost may be justified by reduced SWD damage, especially in small plantings.

The author is a vegetable and berry specialist with University of Vermont Extension based at the Brattleboro office. He can be reached at vernon.grubinger@uvm.edu.