Another animal invader entered the United States, in California in 2009, and has exploded across the country, reaching Beltrami County just this summer.
This invader won’t get the headlines and action that zebra mussels has, but you may want to pay attention. The economic damage is not inconsequential. In New York alone, on a single crop of 500 acres of raspberries in 2010, losses reached 80 percent, a $3 million dollar loss. This does not account for the home gardeners who lost food for their tables and freezers.
Who is this invader? The spotted wing drosophila (SWD), Drosophila suzuki, a native of Asia, is a 2-3 millimeter fly that affects raspberries, blueberries, blackberries, grapes, stone fruits and other soft-bodied fruits. There is nothing like picking a juicy, red raspberry, looking inside only to see wriggling, white maggots already enjoying the tasty morsel you have been anticipating.
The female emerges one day and starts laying eggs with her saw-like ovipositor the next. She lays up to 350 of them and the active period can last up to nine weeks. The eggs hatch in 12 to 72 hours, becoming those little white larvae. The three-larvae stage lasts five to seven days before pupating for another four to five days, becoming the adult fly. Picking and destroying ripe fruit is crucial to prevent exploding numbers of the flies and their offspring.
Can you spray for this pest is often the first question asked. Yes, you can and both nonorganic and organic options are available. Traps must be set early to detect the fly’s presence. Once the eggs are laid, there is no recourse. Costs, economic, ecological and time-wise are associated. Organic sprays must be done every four to five days; nonorganic sprays can be used less frequently. These all take time, money and some can incur risks to our health and to the health of pollinators.
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There are other aids to control spotted wing drosophila:
Frequent (daily) harvest of ripe berries with affected fruit being put in clear plastic bags to be frozen or baked in the sun to eliminate the insect.
Pruning of the canopy of the stand of fruit, increasing sunlight and making it less hospitable to the insects as well as making spray coverage more effective.
Allowing soil surface to dry out between watering and avoiding overhead watering.
Many of our wild and garden species are host plants: honeysuckles, elderberries, dogwoods, blueberries, asparagus, baneberries, plums, choke cherries, choke berries, wild raspberries, blackberries, grapes, as well as weeds and other landscape plants. Eliminating all these species is not practical nor is it desirable as many are ones we enjoy and ones that provide for other species.
Whether the SWD winters over in northern Minnesota is not known. Temperatures colder than 28 degrees and warmer than 91 degrees kill off 75 percent of them, but little is known about the other 25 percent.
As more research is done, more effective methods to deal with this pest will hopefully be found. Some years may be better than others. Perhaps they will decline naturally. Not a lot is known. Weather changes due to global warming along with the global travel and commerce are leading to more challenges that we as a society face. As gardeners, all we can do is practice good growing methods that support the healthy ecology of our local species, soils and water and continue to educate ourselves.
For more information, the University of MInnesota Extension offers information at www.extension.umn.edu/garden/yard-garden/> and a second site in a state with more experience with this insect can be found at Cornell University’s Extension site www.fruit.cornell.edu/spottedwing/> . Minnesota’s site offers information about spraying that is approved in our state.
