Good in the Lab

National Fruit Fly Month — October — has drawn to a close. (That designation is my own, not the federal government’s.) Sure, a few still flit about here and there. But no longer do clouds of them hover over bowls of fruit in my kitchen. In case you haven’t experienced them, fruit flies, Drosophila species, are cute little (about 1/8 inch long) flies that feast on overripe and damaged fruit as well as other plant material.

I, and perhaps you, were first introduced to fruit flies in middle school biology, raising them on some mix of banana and agar-agar. In those days, I got more intimate with them as part of a science project: My project was to test whether x-rays cause mutations. My dentist agreed to help. After raising a batch of flies, I went to his office and laid a vial of them on the dental chair whereupon Dr. Golden zapped them with a beam of x-rays. After the buggers and a similar vial of non-irradiated buggers had reproduced, I examined the offspring, especially their eyes and their wings. My experiment “proved” x-rays benign (as least as far as could be detected morphologically by a 13-year-old).

Fruit flies’ fecundity is what has earned them prominent positions in science. An adult lives for only about a month but during that tenure she may lay as many as 500 eggs. The courtship ritual of those little guys and gals is intricate, involving a kind of dance along with some instrumentation, provided by sounds from leg tapping, and some singing. Each gal gets involved with a number of guys.

Fun fact: Despite the small size of fruit flies, they have the longest sperm cells of any known organism — over 2 inches long for D. bifurca. Most of that length is the sperm’s tail. During mating, those tails are wound up in tightly tangled coils.

Not So Good in the Kitchen

Fruit flies are very interesting and useful in the lab but not in the kitchen. The flies lay eggs on the surfaces of fruits and vegetables. The eggs hatch into larvae which feed, in time turning the fruits and vegetables into a slimy mush. After molting twice, they morph into adults, who flit away to generate more offspring.

Growing up, I only knew fruit flies from the lab, never my mother’s kitchen. Why do they invade my kitchen? One possibility could be the smorgasbord my garden provides, more than ever spread on counters of that kitchen of yore. Also, refrigeration drastically slows the flies’ development but also can suppress or ruin flavors (tomatoes, for example) or speed wilting (lettuce, for example); I only refrigerate for longer term storage, not usually for fresh eating.

And being home grown, my fruits and vegetables need not aspire to the same cosmetic standards as commercial fruits and vegetables. Small wounds that I ignore or cut away provide access to fruit fly activity.

Still, I can’t let the flies run amok. Fortunately, fruit flies are easy to trap. And one of their favored baits, vinegar, is readily available. (Fruit flies have also been called “vinegar flies.”) Just put out a glass of vinegar and you’ll soon see a crowd of fruit flies flocking to it. That, of course, doesn’t trap them.  Make that glass of vinegar into a trap by adding a little soap or detergent to the vinegar to decrease its surface tension so that, once in the drink, the fly can’t fly out.

Various kinds of vinegars are effective. Some people recommend apple cider vinegar. Or apple cider vinegar with a slice of banana thrown in. My preference is for balsamic vinegar. A mix of sugar, yeast, and water also works.

Look on the web and you’ll see all sorts of more intricate — but none very intricate — traps for fruit flies. My method is to take a plastic carryout container with a lid, punch or drill a 1/8” wide hole in the lid, pour some balsamic vinegar into the container, then replace the lid.

A more Frightening Relative

Spotted wing drosophila

October isn’t the only month of prominence for fruit flies. They are out there whenever the weather is congenial and food is available. One of them, spotted wing drosophila (SWD) is fairly unique in that this species need not wait for a fruit to be overripe, damaged, or nearly rotting to attack. It threatens intact, even underripe, fruit such as raspberries, blueberries, and other fruits of summer.

SWD is a relatively recent arrival on the garden scene (2008 in California, 2013 in my garden). At first it seemed that was going to be the end of blueberries, my favorite fruit. Over the years, I’ve been able to mitigate damage. Early on, I tried sprays of Entrust and Neem, as per recommendations. But even though organically approved, for me these sprays take some of the enjoyment out of growing and eating the berries. Fine mesh covering over a whole planting has also been tried (effective, but not tried by me).

This year, and for the past few years, my blueberry planting has yielded its usual, good harvests utilizing a multi-pronged approach. For starters, I grow a dozen or so varieties that extend the season from late June until early September. SWD, here, at least, is inconsequential until early August, by which time our bellies and the freezer are almost full of berries.

In the latter part of July, I hang out traps, one per bush, that I’m helping test for Cornell’s Peter Jentsch. They are baited with raspberry essence laced with boric acid, which needs to be renewed by weekly spraying (of the trap, not the plants). After 3 years of testing, the traps seem to be effective. 

During summer, there’s always a bowl of freshly picked berries on the kitchen counter; by early August, fresh picked fruit goes into the refrigerator or, if the freezer’s quota still not yet filled, also into the freezer. Any larvae within fruit are killed after 3 days of refrigeration.

Fruit flies and even SWD, despite their fecundity, aren’t about to take over the world. These pests have their pests, which include yellow jackets, ant, various other kinds of flies, and even smaller creatures, such as nematodes. Quoting Jonathan Swift, “. . . a flea, Hath smaller fleas that on him prey; And these have smaller fleas to bite ‘em, and so proceed ad infinitum.”