Orchard owners and backyard fruit growers are facing a formidable enemy this season in the form of a tiny fly, spotted wing drosophila (SWD), or drosophila suzukii. It’s tiny but mighty, able to lay eggs through unbroken skins of fruit such as cherries, blueberries, etc. There are about 1,500 species of drosophila, and this is the only one that attacks whole living fruit. We know the other species as fruit or vinegar flies, those that further the decay process of rotting fruit as they feed.
This fly is new to the U.S., having appeared first in California in 2008 and in Idaho in 2012. It’s native to Japan and Korea. Entomologists on the West Coast are scrambling to learn all they can about these insects’ lives, because when one knows all crucial information about an insect, one can use that information to help control it.
When fly eggs hatch, the larvae are white maggots, and fruit containing those are not salable in commerce. Some California fruit growers have suffered 20 percent losses due to this mite of a fly. Home gardeners are also at risk from this fly, losing even bramble fruits to their egg laying. We already have problems controlling cherry fruit fly maggots, and certainly don’t need maggots in raspberries and blackberries.
What can we do? Apparently they’re attracted to yellow or red, so we could make traps using lidded paper cups with at least some of either color on them. Holes in or near the top of the cup should be 3/16 of an inch in diameter. If holes are larger, you may trap beneficial insects unintentionally. Even so, you may trap some of the tiny beneficial wasps such as Encarsia species. To attract insects, use apple cider vinegar with a bit of dishwashing liquid to break surface tension. If you don’t break it, flies drink and fly away. If you’ve added a bit of dishwashing liquid, they land, sink and die.
I’d suggest using raw unpasteurized apple cider vinegar from the health food section of the supermarket rather than the pasteurized Heinz brand. Or use a mixture of dry baker’s yeast (two tablespoons), sugar (4 teaspoons) to one and a half cups of warm water. Hang these homemade traps in or near fruit trees or shrubs, and monitor them. You’d probably need a microscope to identify a female (and know the shape of the ovipositor you’re looking for), but the males have a black spot near the far edge of each wing. If and when there’s a buildup of bodies, wash it out and refresh the apple cider vinegar.
Can you trap enough to make a difference? I think we can. The numbers of this fly apparently fluctuate by season, large numbers in spring, fewer in midsummer, and a very large increase in numbers in late summer to early autumn, according to experiments conducted in California. Idaho entomologists are still trapping them, trying to figure out what they’re feeding on in winter here. They’ll broach and lay eggs in any fruit.
Insects are drawn to colors and can also be trapped by painting colored plates or cups with a product called “Tanglefoot.” Note the intensity of the yellow on insect-trapping cards sold by garden stores. The same hue of color is available in some paper (or foam) plates. You can make your own traps by painting Tanglefoot on those plates if you fix a way to stake them or hang them near vulnerable crops. These are irksome to humans who accidentally touch or bump one of these traps, but we’ll learn. They unfortunately may trap beneficial insects, too.
You could also trap thrips using this technique. Thrips are attracted to a shade of blue I haven’t noticed on other items, but haven’t searched for those items. The blue cards available in garden stores trap thrips drawn to that color instead of yellow.
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