Wednesday, March 4, 2015

Spotted Wing Drosophila

Red raspberries are under attack.
New food pests are never welcome, but the spotted wing drosophila will prove to be particularly problematic. This fruit fly invades soft skinned fruits -- pretty much all berries, cherries and peaches. I imagine they'll also attack apricots, perhaps even plums. Anything with a soft skin. Apples and pears produce skin too tough for the fruit flies. Even tomatoes aren't entirely safe. While their skin may be too tough, the spotted wing drosophila (SWD) will take advantage of cracks.

Fruit flies tend to be a problem only on rotting or fermenting fruits, so don't generally present many issues in the fruit garden. However, SWD attacks ripening fruit, laying its eggs inside the fruit, so its eggs and larvae are safe from predators and pesticides.
Spotted wing drosophila likes peaches, too.

While home gardeners can just decide to ignore an egg or larvae in their berries, those producing these fruits for sale are scrambling to figure out ways to control them. Consumers don't like buying a box of blueberries and finding little fruit fly maggots in some of the berries.

Control of the SWD starts with sanitation. Don't leave any fruit in the berry patch. When you pick, take a basket for the good berries and a basket for the bad berries. Don't just toss the bad berries in the compost heap. Tests have shown that you must bury those berries at least a foot deep to prevent the larvae from pupating and taking flight. You can put the culls in a clear plastic bag and set it in the sun for a few days to kill the eggs and larvae, or you can stick them in the deep freeze for a few days. Pruning that opens the "canopy," thus creating less shade, can help, as well.

Grapes aren't immune.
Because the eggs and larvae are inside the fruits, pesticides work only on the adult flies. Several conventional pesticides work on them, as well as a couple of organically approved pesticides. Look for products containing spinosad, a natural insecticide produced by soil microbes. Pyrethrum products (made from a certain daisy species) also work, but pyrethrum  also is toxic to bees, which our berries need for pollination. So great care must be taken when applying such pesticides. The downside of these two organic pesticides is that they are short-lived and must be applied about once a week. The upside is that they deteriorate quickly and you won't be eating toxic chemicals on your berries.

Adding sugar to the pesticide mixture encourages the fruit flies to eat it. The presenter at the Missouri Organic Association conference recommended that you alternate pesticides -- pyrethrum one week, spinosad the next -- to avoid pesticide resistance in the fruit flies.

I asked if Surround -- a slurry of kaolin clay that coats fruit and leaf surfaces to repel insects -- works against SWD. The answer was that it would work, but the white coating is difficult to wash off of the soft-skinned berries. Kaolin is non-toxic -- it is present in toothpaste and other cosmetic products -- so eating a little is no problem for the home gardener. However, it presents aesthetics issues for the market gardener.

Early fruits -- such as black raspberries and early strawberries and blueberries -- are far less susceptible because they are harvested before SWD usually shows up in mid- to late June. However, weather that warms early could result in early arrival. Before wholesale application of pesticides, monitor your berry patches to determine whether SWD is present and when it shows up. Otherwise, you are wasting money on pesticides and unnecessarily endangering pollinators.
Late season blueberries are susceptible.

Since berries continue to flower while their fruit is harvested, it is important to be conscientious about the bees. A trap for monitoring for SWD can be made from a plastic container with holes punched around the top that is hung in a shaded area among the berry plants or susceptible tree fruits. The best bait for these traps is 1/2 tablespoon active dry yeast mixed with 2 tablespoons sugar and 6 ounces of water per quart-size trap. The fruit flies crawl in to get at the bait and drown. You will need magnification to determine whether the drowned fruit flies in your trap are SWD or some less destructive fruit fly. Your local Extension agent can help you identify them.

Even late season strawberries are targets of SWD.

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