Tuesday, September 18, 2012

Could a Cherry Tree Tent be the Answer?

 We've posted before about a huge threat to backyard cherry trees in our area: the cherry maggot, also known as the Spotted Wing Drosophila. SWD is responsible for the little worms that backyard gardeners are starting to find in their cherries, and to a lesser extent in raspberries, strawberries and blueberries. An experiment conducted at the Fair Oaks Horticulture Center this past year by Sacramento County Master Gardeners may provide an answer to this growing problem throughout the West.

This was originally published in the Sacramento County Master Gardener Newsletter of Sept. 2012.
 
The problem: Spotted wing drosophila (SWD), is a gnat-sized fruit fly first found in 2008 damaging fruit in many California counties. Unlike the common vinegar fly that attacks rotting and fermenting fruit, the SWD infests ripening, undamaged cherries as well as ripening raspberry, blackberry, blueberry, and strawberry crops – especially in coastal areas.
 

Prior to 2011, no SWD damage was found on the multi-grafted cherry tree at the Fair Oaks Horticulture Center. In 2011, the early varieties escaped damage, but the later varieties were totally infested. In an attempt to prevent fruit loss in 2012, and to demonstrate to homeowners a possible cultural means of preventing infestation, the orchard group decided to tent the tree.
 



The tent: Mary Kay Ryan and Patty Peterson designed a structure to cover the tree while still allowing the harvest of seven varieties of cherries ripening at different times. The tree was pruned to just under 10 feet. Mary Kay and Patty used 1” schedule 40 PVC to create a large box frame anchored to the ground at the corners. The tent itself – 13.6 feet wide, 13.6 feet long, and 10 feet high – was sewn by Patty using 30-weight Agribon row cover and included a 110” sleeping bag zipper to allow easy entry. Diagonal guy wires and PVC clamps ensured the tent would not slip or be pulled due to winds. The bottom edge of the tent was weighted down. The tent was erected 4 weeks prior to the first harvest. The cost of the tent: approximately $200.
 

Traps: One SWD trap was placed outside the tent, and one trap inside the tent to monitor activity. A few male SWDs (with distinctive wing spots) were found in the outside trap during the season, but an exact number of SWD males was not determined, as there were many similar small insects in the trap. No SWD males were caught inside the tent.
 

Temperature observations: Because we were concerned about heat build-up in the tent, two recording thermometers were placed in the canopy of the cherry tree at 6 feet height under the Agribon from May 14 through June 19. Two additional thermometers were placed at the same height in an uncovered fig tree of similar size.
The low (nighttime) temperatures were on average 3 degrees cooler in the tree under the Agribon than in the uncovered tree (see graph). The high (daytime) temperatures under the Agribon were on average 2 degrees warmer than the uncovered tree. On days over 90F, temperatures under the Agribon were still only 2.5 degrees warmer, but during one 3-day period, temperatures averaged 8 degrees warmer.
 

Harvest: While harvest was a bit challenging (quickly entering/exiting the tent as well as moving ladders during harvest in a cramped space), overall it was a success with a full crop of undamaged, delicious cherries!
 
Other issues/drawbacks: The size of the tent required a number of us, with ladders, to assist in its assembly. The tent did suffer some damage near the bottom – we believe from a raccoon or similar creature. Several tears appeared overnight but were quickly repaired. Additionally, the tent suffered some wind damage along the bottom edge. We originally used 2’x4’s to secure the tent bottom, switching to soil to prevent further tears. On the plus side, we were able to disassemble the tent relatively quickly and store the parts for next season.
 

Conclusion: While our experiment was certainly not scientific, we do think of it as successful in that we avoided fruit damage of any kind on a fairly large tree with varying harvest periods. It does provide another option for homeowners who may be dealing with SWD. Although expensive initially, the cover should be reusable for possibly several years. In January 2013, we will espalier a cherry tree, and when the tree begins bearing we will secure row cover over it in the spring. This will provide demonstrations of both training and pruning on a trellis and a practical method of controlling SWD, since covering an espalier tree should be much cheaper and easier than a large tree.
—Tracy Lesperance, Cathy Coulter, and Chuck Ingels



For more information about the Sacramento County Master Gardener Program, including links to their wonderful 2013 Gardening Guide and Calendar, click here.

3 comments:

  1. Wow. That is so interesting. A lot of work and expense for one cherry harvest but good to know.

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  2. $200 for atree's worth of bug- and bird-free cherries? Probably worth it. But my tree is about 20 feet tall, and I really don't want cut it back. *sigh* guess I'll keep looking and hoping ...

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  3. I'd like to see the design plans for this, Fred. I've always hated netting my cherry tree because it bends the branches terribly. I do it, but I've branches growing in horizontal directions, which is never a good thing.

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