Fruit Fly Secrets: How We Produce Booming Cultures

By Allen Booth and Derick Michael

Allen has 30+ years in the exotics industry. Derick runs MLE's dart frog breeding program in New Jersey.

We wrote a piece a while back about what to do when you run out of fruit flies. This is the other half: how to make sure it stops happening.

Here is the thing nobody tells you. Almost everything written about fruit fly cultures is about the recipe. What media, what ratio, what yeast, what magic ingredient. People go down that rabbit hole for years.

The recipe is not the answer. Once you have one that works, the recipe is close to the least interesting part of this. Everything that actually determines whether you have flies on feeding day happens around the recipe: where the cultures sit, what light hits them, what you mix into new ones, and how much margin you keep.

What I got wrong

For the first few years I crashed cultures constantly. Looking back, I did none of what is in this article.

• I kept everything in one spot. New cultures, producing cultures, spent cultures, all together. Which means when mites showed up, they showed up everywhere.
• I didn't have a formula locked down. I was still fiddling, which meant I never knew whether a bad batch was the media or something else.
• I ordered from different places. Different sources, different quality, no baseline. Nothing was ever a controlled variable.

Everything below is what replaced that. None of it is complicated. It is just discipline in places I did not know mattered.

Setup

Start with someone else's mix

There are a lot of formulas out there. If you are a beginner, buy a mix you know is good. Plenty of companies make them, including us. Get consistent yields first.

Once you are producing regularly and getting good yields, then start working on your own mix. But do not wait until you are out of the stuff you have been buying. That is how you end up in a bad spot. Start experimenting while you still have good premade mix producing in the background, so a failed experiment costs you nothing.

Then, once you find a formula that works, purchased or your own: stick with it. We are not going to get into recipes here. That is a rabbit hole and it is not where the wins are.

Media amount and water

Too little media is a problem. Too much media is not. Do not be scared to add slightly more than the formula suggests.

Water is where the real risk is. If you add too much water the culture goes too soft, and you end up with liquid at the bottom when you pour. Once that happens the culture is genuinely hard to use, even if it produced flies. You learn over time how much is too much, and getting it slightly dry is a much smaller mistake than getting it wet.

Excelsior and surface area

More surface area means more flies. That is the entire principle, and it is why we fill our cultures with excelsior almost to the top. The larvae and flies need somewhere to go, and the more of it you give them, the bigger the boom.

We buy excelsior in bulk now, so that is what we use. Before that we used coffee filters, and they work fine. If you are keeping costs down, coffee filters are a real option, not a compromise you should feel bad about.

The operational stuff (this is where the wins are)

1. Mix old cultures in, not just the booming ones

When we make new cultures, most people take their newest booming culture and shake flies from it. We do not do that, or at least not only that.

The rule: old cultures first, booming ones second. We keep some of the older cultures around, the ones past their peak that are still putting out some flies, and we mix whatever we can get from those in first. Then we top up from the booming ones. Some days that is a lot of old flies, some days it is a few. There is no exact number, but we always do it.

Why it works, as best we can tell: you get flies at different life stages and a more diverse genetic pool going into the culture. Newly-producing flies take time to start producing themselves. Mixing in older ones seems to get the larvae going faster.

2. Separate your cultures into zones. This is a big one.

Our cultures move through three different spots in the room as they age. Not adjacent shelves, genuinely different spots, away from each other.

• Spot 1: new cultures, waiting to produce.
• Spot 2: once a culture is booming (lots of flies), it moves here.
• Spot 3: once we have used a good amount of it and we are waiting on the second boom, it moves again.

The point is mites. If the old cultures never sit near the new ones, mites cannot walk from one to the other. We have never had mites take over the new cultures since we started doing this. It costs nothing but discipline and it is probably the single highest-value thing on this list.

3. Light. Flies want darkness.

We found this one the hard way.

We had a window that let in some direct light in the mornings. It was not even hitting the cultures directly, just nearby, and only for a little while each day. The cultures produced, but they never boomed. We could not work out why.

We covered the window and it was a different story. Booms and booms. The flies clearly prefer darkness to light.

We have not tried complete darkness and we are not going to tell you to, since we have not tested it. But finding a spot away from the sun and away from your terrarium lights was a real turning point for us.

4. Room temperature

The good news is that fruit flies want roughly what we want and what our frogs want: about 72 to 78 degrees Fahrenheit (22 to 26 Celsius).

The closer you get to 75 to 77 F (24 to 25 C), the better the yields. If you can hold that zone you are in a good place.

Above 80 F (27 C) they slow down, and the higher you go the more likely they crash. That is not one culture, that is potentially all of them at once, which is exactly the scenario worth engineering against.

We have never dealt with really cold, because we run backup generators. So we cannot tell you where the bottom is.

The feeding workflow

The bucket rig (fewer escaped flies)

We are not going to pretend we have solved flies escaping from terrariums. If you have figured that one out, please tell us. But we have cut way down on the ones that escape during feeding, and this is how.

• Start with a bucket. A Home Depot blue bucket is what we use.
• Put three deli cups inside it.
• On top of those, place the deli cup you are actually going to feed from.
• Pour your vitamins into the top cup.
• Take a culture, shake it up, and only peel back a small section of the lid, do not take the whole lid off. Shake it into the top deli cup. Repeat for the next culture.

Anything that misses falls into the three cups below, which you just shake into the main cup at the end. A few will miss everything and end up in the bucket itself. Take those outside and shake them out. Those flies are free. It is normally just a few, but if you leave them they will climb out.

Measuring at the terrarium

When we feed, we do not just open the door and pour from the deli container. It is very easy to overfeed that way. We shake some flies into a small plastic container first to get the measurement, then put them in. It makes it easier to place the flies where you want them and much harder to overdo it.

And do not forget the banana trick. If you have read the other article you already know what that is.

How we read a culture

Larvae climbing is the main tell. When you see lots of larvae climbing, things are going well.

The batch comparison is the real trick, and it only works because we make cultures in batches. Everything made on the same day should be doing roughly the same thing. If one culture is behind its siblings from the same date, it is visible immediately. You have a built-in control group.

Here is the part that surprises people: we do not do anything about it. A culture that is lagging will still probably produce some flies. It is not a crisis and it does not need rescuing. It just is what it is, and there is plenty else producing.

The too-wet tell: if you put in too much water the culture will probably smell, and it might still make flies, but when you go to pour it the liquid comes out and it is really not usable. You learn over time how much water is too much.

The point of all of it

Look back at that list. None of it is a clever recipe. It is: keep old and new apart, mix broadly, keep it dark, hold your temperature, and always make more than you need.

The reason I do not crash anymore is not that I got better at predicting failures. It is that I built a system where one failure does not matter. There is always another culture coming. That is the same test from the other article: when you are done feeding your frogs and done making new cultures, do you still have flies left over? If yes, a crash is an inconvenience. If no, a crash is an emergency.