Fruit flies and your sourdough starter: covers that work and a feeding-day routine

Fruit flies show up at feeding time because your starter is making the exact smells they follow: fermentation acids, esters and live yeast. You don't have to choose between a healthy starter and a fly-free jar. The fix is a cover that breathes but doesn't let anything in, plus a counter that doesn't smell like dinner to a fly.

Why a starter is a fruit fly magnet

Fruit flies find food by smell, and the smells that matter most are fermentation smells. In lab work, fermenting yeast alone was enough to attract common fruit flies, trigger egg-laying and support larvae (Becher et al., 2012). Acetic acid, which builds in a sourdough starter between feedings, attracts flying fruit flies on its own (Becher et al., 2010). A warm, open jar of active starter is broadcasting both.

What actually happens if they get in

A female fruit fly can lay about 500 eggs in her lifetime, often in moist, fermenting material (University of Kentucky, ENTFACT-621). A starter is moist, fermenting material. If you find larvae in the jar, the starter is compromised: discard it, wash the jar in hot soapy water, and rebuild from a backup. This is why every serious baker keeps a dried or frozen backup.

Covers that breathe but don't let flies in

Adult fruit flies are about 3 mm long, so a cover has to be finer than that, with no gaps at the rim.

• Works: a tight-weave cotton cloth or a coffee filter held with a rubber band; a lid with a fine mesh insert; a jar with the lid screwed on loosely enough to vent, which lets gas out but gives flies no straight path in.
• Doesn't work: cheesecloth (the holes are bigger than a fruit fly), a plate set on top (gaps at the rim), a towel draped loosely.

Rinse the rim and the outside of the jar after each feeding. Starter smeared on the glass is bait sitting right next to the cover.

The feeding-day routine

1. Feed the starter, then wipe the rim and outside of the jar clean.
2. Put the discard in the fridge or straight into a recipe. A bowl of discard on the counter is a second starter as far as a fly is concerned.
3. Rinse the spoon and any flour-and-water paste off the counter. Dried paste is fine; wet paste is food.
4. Cover and move the jar away from the fruit bowl and the sink. Those two spots are where fruit flies already gather.

Keep the counter from inviting them

Once the jar is covered, the rest is about the room. Fruit flies breed in drains, disposals and trash as readily as in fruit (Purdue Extension, HN-66), so a starter problem is usually a kitchen problem that found the starter.

• Drain. Brush the drain throat, run the hot tap, then add Sink Drops. Three nights in a row, then two or three times a week.
• Counter. Set a peppermint and clove disc a few inches from the starter jar and another by the sink. In a lab study, peppermint oil and its compounds menthone and menthol repelled the common fruit fly (Wang et al., 2021). The discs don't touch the starter and don't affect fermentation; they change what the counter smells like. The study tested the ingredients, not our discs, so use your own kitchen as the test. You have 30 days.
• Fruit. During an outbreak, keep ripening fruit in the fridge for a week.

What not to do

• Don't spray anything near the jar. Your starter is a culture; it will pick up whatever is in the air around it.
• Don't set a vinegar trap next to the starter. Vinegar is an attractant. A trap beside the jar brings flies to the jar.
• Don't leave the starter out between feedings if you bake weekly. Keep it in the fridge and bring it out the day before. Cold slows both the starter and the flies.

Sources

• Becher PG, Flick G, Rozpędowska E, et al. (2012). Yeast, not fruit volatiles mediate Drosophila melanogaster attraction, oviposition and development. Functional Ecology 26:822-828. doi:10.1111/j.1365-2435.2012.02006.x
• Becher PG, Bengtsson M, Hansson BS, Witzgall P (2010). Flying the fly: long-range flight behavior of Drosophila melanogaster to attractive odors. Journal of Chemical Ecology 36:599-607. doi:10.1007/s10886-010-9794-2
• University of Kentucky Entomology, Fruit Flies, ENTFACT-621. entomology.mgcafe.uky.edu/ef621
• Purdue Extension Entomology, Fruit Flies, HN-66. extension.entm.purdue.edu
• Wang Q et al. (2021). Behavioral and physiological responses of Drosophila melanogaster and D. suzukii to volatiles from plant essential oils. Pest Management Science 77:3698-3705. doi:10.1002/ps.6282