Keeping Black Soldier Fly Larvae

Black soldier fly larvae are sold under a lot of names, and almost all of the marketing is about calcium. Less gets said about the fact that they are on a clock, that the clock is pupation rather than death, and that the temperature they hold best at is not the temperature anything else in your feeder cupboard wants. This guide covers holding a batch of soldier fly larvae after it arrives.

The container, and the one behavior that decides it

Soldier fly larvae are placid until they are not. At the end of larval development the animal empties its gut, stops feeding, darkens, and goes looking for somewhere dry to pupate. That migration is deliberate and well documented, and commercial producers build ramps to exploit it, because prepupae will climb out of a container on their own.

So the container is a smooth-sided tub or cup with a secure, vented lid. Not a bowl. Not a cup with a loose lid. The escape risk is near zero for pale, feeding larvae and rises sharply the moment they start going dark.

Larvae arrive in their own spent substrate and frass, and that is the right thing to hold them in. A 2025 storage study found that adding frass improved larval survival and allowed longer storage, so the instinct to tip them into something clean is the wrong one. Leave them in what they came in, and do not add moisture. The same study found that storing larger quantities together, and storing in bags, also improved survival, and that longer storage cost the larvae weight.

• Vented lid, secured. Small holes, not an open top.
• Do not rinse them or add water. Wet substrate is a spoilage risk and it pushes them to migrate.
• Do not add fresh food. More on this below.
• Check for darkening. Dark larvae are prepupae, and they are on their way out of the feeder stage.

Temperature: the one feeder that wants to be cold

The published thermal biology here is unusually good, and it splits cleanly into a growing temperature and a holding temperature.

For growth, a 2018 study across nine constant temperatures put the lower developmental threshold for larvae at about 12 °C and identified 30 °C as the most favorable temperature, with an upper lethal threshold somewhere between about 37 and 44 °C. Rearing work from 2009 found that survival of young larvae through to adult averaged 74 to 97% at 27 and 30 °C but collapsed to 0.1% at 36 °C. The working window is genuinely narrow at the top end, and a closed container left in a warm car will find it.

For holding, you want the opposite. A 2025 study on storing live larvae tested 6, 8 and 10 °C and found 10 °C gave the longest usable life, with mortality under 15% for up to about six days. Ten degrees Celsius is 50 °F: a wine cooler or a cool cellar, not a fridge and not a shelf.

What about the fridge?

A domestic fridge runs colder than that, usually around 3 to 5 °C. A 2019 study on cold tolerance in immature soldier flies held them at 4 °C and found the answer depends entirely on how big they are. After 72 hours, fifth instars survived at about 92% and third instars at about 81%, but second instars survived at 2% and pupae at about 23%. Below freezing there is no story to tell: at −12 °C essentially nothing survived an hour.

The honest reading is that a fridge will not immediately kill a batch of large larvae, but it is colder than the published storage optimum, small larvae do badly in it, and the damage accumulates. If you have somewhere around 50 °F, use that. If the fridge is your only cold option, treat it as a short-term measure rather than a storage plan, and keep them out of the coldest part.

Feeding and watering: mostly, don't

This is the guide's easiest section. You do not feed or water soldier fly larvae while holding them. They arrive with residual substrate, and the last larval stage stops feeding altogether before it pupates.

Adding fresh food to a holding container gets you wet substrate, which gets you spoilage and premature migration. Adding water gets you the same thing faster. If a batch looks dry and dusty, that is the substrate doing its job.

The trade-off is that you cannot gut-load them the way you would a cricket or a roach. What you buy is what you feed out. If gut-loading is part of your routine, a different feeder carries it better. Our gut loading guide covers which feeders respond best, including the limited vitamin A work on soldier fly larvae.

The calcium claim, stated properly

Soldier fly larvae are genuinely the exception among common feeders. They are the one commercially produced feeder insect reported to have a naturally favorable calcium-to-phosphorus ratio, often quoted as around 2.5 to 1. That part is real, and it is why people buy them.

Two qualifications that rarely make it into the marketing:

• The ratio moves with what the larvae were reared on. Published calcium figures for this species span a wide range across studies, because substrate and processing both shift it. Any single percentage quoted at you is one study's larvae on one diet.
• Total calcium is not available calcium. This is the one that matters. In a feeding trial with leopard geckos, calcium digestibility came out at about 43%, because the calcium-rich exoskeleton passed through largely intact. An earlier trial with mountain chicken frogs came out lower still. Our black soldier fly larvae guide covers the gecko study.

The conclusion is not that soldier fly larvae are a bad feeder. It is that "high calcium, so no dusting needed" does not follow from the evidence. If you are managing an animal with a diagnosed calcium problem, that is a conversation with an exotics vet, not something a feeder choice settles. Keepers of leopard geckos, frogs and other amphibians in particular should treat the ratio as a starting point rather than a supplement plan.

How long a batch keeps, and what ends it

Note what is not in that table: dying of old age. The thing that ends a batch as a feeder is almost always pupation, not mortality. Darkened prepupae are still alive. They have simply stopped being what you bought.

What goes wrong

Fewer things than with crickets, and they are mostly temperature.

• Too warm. The batch runs through to prepupae in days rather than weeks. Not a failure exactly, but you lose the window.
• Too cold. Freezing kills quickly. Sustained fridge temperature is survivable for large larvae and much less so for small ones.
• Too hot. Survival collapses in the mid-thirties Celsius. A closed container in a warm car does this easily.
• Wet substrate. Spoils, smells, and drives the larvae to climb out looking for somewhere dry.

When to write off a batch

Give up when the substrate is genuinely sour and wet through, or when the container smells of decomposition rather than of damp bran. Dead soldier fly larvae go soft and dark and the substrate goes with them, and there is no rescuing that.

Do not write off a batch just because it has gone dark. Prepupae are not dead, they are past the feeding stage, and some keepers feed them out anyway, accepting that the animal is now harder-shelled and no longer feeding. If the whole batch has darkened at once, that is a temperature story, and the next order wants to go somewhere cooler.

A batch that arrives already dead, mushy or foul is an arrival condition and not something a cool shelf fixes. Our guide on what to do when your feeders arrive covers what to check and in what order, and our shipping page sets out the live arrival terms, including sending photos or video within 1 hour of delivery if bugs arrive dead.

Where to go next

Soldier fly larvae are worth keeping in rotation precisely because they are unlike everything else in the cupboard: no feeding, no watering, and a cool shelf rather than a warm one. That last part is the awkward bit, because it means they cannot live next to the roach bin. Setting up a feeder storage station covers arranging a cupboard with two temperature zones. The wider feeder insect range is the place to work out what to pair them with, and if the pairing is dubia, the dubia roach size chart matches roach size to your animal.