How to keep fruit flies out of your kombucha

Kombucha makes the two things fruit flies follow most: acetic acid and live yeast. A brewing vessel is open on purpose, so you can't seal it. You can make it impossible to enter and make the shelf around it a place fruit flies don't want to land.

Why they find your brew

Fruit flies locate food by smell. Acetic acid, the main volatile in vinegar and the acid your SCOBY produces as it works, attracts flying fruit flies on its own, and vinegar odor drew 62% of flies in a wind-tunnel test (Becher et al., 2010). Fermenting yeast is enough by itself to trigger attraction, egg-laying and larval growth (Becher et al., 2012). A warm F1 vessel offers both, all day, with the lid off.

What's at stake

A female fruit fly can lay about 500 eggs, usually in moist, fermenting material (University of Kentucky, ENTFACT-621). If larvae appear on the SCOBY or in the liquid, the batch is done: discard the liquid, and unless you have a clean pellicle in a SCOBY hotel, start the culture over. A dried or refrigerated backup turns a bad week into a minor one.

Covers that actually keep them out

Adult fruit flies are about 3 mm long. The cover has to be finer than that with no gap at the rim, and it still has to let the culture breathe.

• Works: tight-weave cotton, a coffee filter, or a paper towel, held with a rubber band or a band that fits the jar; a lid with a fine mesh insert.
• Doesn't work: cheesecloth (holes bigger than a fruit fly), a cloth laid on top without a band, a continuous-brew spigot that drips and is never wiped.

Wipe the rim and the outside of the vessel after every top-up. Kombucha film on the glass is bait sitting beside the cover.

The brew-day routine

1. Bottle or top up, then wipe the rim, the spigot and the shelf.
2. Rinse bottles the same day. An empty kombucha bottle with a few drops in it is a breeding jar.
3. Put fruit for second fermentation straight in the bottles or back in the fridge. Cut fruit on the counter is where flies start.
4. Cover, band, and move the vessel away from the fruit bowl and the sink.

Keep the brewing shelf uninviting

Once the vessel is sealed off, the flies you still see are coming from somewhere else in the kitchen. Drains, disposals and trash are breeding sites as often as fruit is (Purdue Extension, HN-66).

• Drain. Brush the drain throat, run the hot tap, then add Sink Drops. Three nights running, then two or three times a week. Extension entomologists recommend the brush and hot flush because the film in the pipe is what fruit flies lay eggs in (Texas A&M AgriLife, ENTO-050).
• Shelf. Set a peppermint and clove disc a few inches from the vessel and another by the sink. Peppermint oil and its main compounds repelled the common fruit fly in a lab study (Wang et al., 2021). The discs don't touch the brew and don't change the flavor; they change what the shelf smells like. Those studies tested the ingredients, not our discs, so judge it in your own kitchen with the 30-day guarantee behind you.
• Bottles and caps. Store rinsed and upside down.

What not to do

• Don't put a vinegar trap next to the vessel. Vinegar is the attractant. You'd be adding a second lure beside the first.
• Don't spray near the brew. A live culture takes in whatever is in the air around it.
• Don't ignore the spigot. On continuous-brew setups it is the one spot that is always wet and sweet. Wipe it daily.

Sources

• 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
• 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
• University of Kentucky Entomology, Fruit Flies, ENTFACT-621. entomology.mgcafe.uky.edu/ef621
• Purdue Extension Entomology, Fruit Flies, HN-66. extension.entm.purdue.edu
• Texas A&M AgriLife Extension, Indoor Flies and Their Control, ENTO-050. agrilifeextension.tamu.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