Black soldier fly larvae get recommended as the feeder that fixes the calcium problem, and mealworms get dismissed as the one that causes it. Both statements are too simple. This guide covers what each larva is, how they size up, what the calcium claim actually rests on and where it breaks, how each behaves once it is in front of an animal, and what keeping them costs you in effort.
What each one is
Black soldier fly larvae are the larval stage of Hermetia illucens, a stratiomyid fly. They are sold under several trade names depending on the producer, but it is one species. Larvae reach about 27 mm at the end of the larval stage. The adult is a 13 to 20 mm fly with no functional mouthparts, no sting, and no digestive organs; it does not feed as an adult. That matters if you are worried about what happens when a tub is forgotten in a warm room.
Mealworms are the larvae of Tenebrio molitor, a darkling beetle, and have been the default cheap feeder for decades. They are a beetle grub in bran, and the forgotten-tub outcome is beetles.
We sell both black soldier fly larvae and mealworms, and both are worth having. They are not interchangeable.
Size and how each scales
Neither is a large feeder. Mealworms cover more of the small end usefully, and are graded down to sizes suitable for hatchlings; black soldier fly larvae are typically offered in a narrower band of small and medium grades. Neither is a meal for a large adult lizard on its own, which is why both are usually part of a rotation rather than the whole diet.
The practical ceiling is different too. Mealworms have giant grades. Black soldier fly larvae do not get much past 27 mm before they stop being larvae. If you need one worm that satisfies a big animal, neither of these is the answer and you want mealworms vs. superworms instead.
The calcium argument, and where it stops working
The reason black soldier fly larvae get recommended is real: they carry substantially more calcium than a mealworm, held largely in the body wall rather than in the gut, which means it does not wash out the way gut-loaded calcium does. For most common feeders, calcium sits well below phosphorus and has to be added by dusting. This species is the exception in the usual feeder list.
The part that gets left out is that the figure moves. Published calcium values for H. illucens larvae span a very wide range depending on what the larvae were reared on, with review data running from roughly 1.2 g/kg to 35.7 g/kg. That is close to a thirty-fold spread. A larva grown on a mineral-rich substrate is a genuinely calcium-dense feeder. A larva grown on something else may not be. "No dusting required" is a claim about a specific production process, not about a species, and no shop can honestly make it as a blanket statement.
They are also high in chitin, and full-fat larvae are fatty. The calcium research, with sources, is in our black soldier fly larvae guide, and the bearded dragon feeder guide has a nutrition table that includes both larvae. What belongs here is the conclusion: treat black soldier fly larvae as a strong contributor to a varied diet, not as a supplement replacement, and keep dusting the rest of what you feed.
Behavior in the enclosure
Mealworms burrow. On loose substrate they are gone in seconds, which is why they suit a smooth-sided feeding dish and animals that will use one. Black soldier fly larvae are slower and stay findable for longer, and their pale, segmented shape gets ignored by some animals and taken enthusiastically by others. There is no way to predict which; if an animal is new to them, offer a few before you order a large quantity.
Neither moves in a way that reliably triggers a hunting response in an animal that only strikes at movement. For a chameleon or a shy animal off its food, both of these are the wrong tool and a moving feeder is the right one.
Black soldier fly larvae will also wander when they are ready to pupate. They darken, stop feeding, and climb. In an open dish in a warm room this ends with larvae on the shelf. This is not a defect, it is the life cycle doing what it does, and it is the main reason keepers run through them faster than they planned.
Keeping them once the box arrives
Open the box as soon as it arrives. The arrival guide covers the first hour; the difference afterward is mostly about temperature.
| Black soldier fly larvae | Mealworms | |
|---|---|---|
| Species | Hermetia illucens, a fly | Tenebrio molitor, a beetle |
| Maximum larval length | About 27 mm | Smaller as standard grades; giant grades exceed it |
| Feeding while you hold them | Arrive in their medium; generally need nothing added | Bran or oats plus a vegetable slice for moisture |
| Refrigeration | No. Cool storage slows them, cold damages them | Yes. Cold storage near 10 °C is an established holding method |
| What happens if you forget them | They pupate, then hatch into flies that cannot bite, sting or feed | They pupate, then become beetles that lay eggs and continue |
| Calcium | High, but dependent on rearing substrate | Low relative to phosphorus; needs dusting |
| Escape behavior | Wandering pre-pupal larvae climb | Burrow down and disappear into substrate |
| Smell | Low if the medium is not wet | Low |
The maintenance detail is in the guides to keeping black soldier fly larvae and storing mealworms and superworms.
Florida keepers
Hermetia illucens is established in Florida, and the state's Department of Agriculture and Consumer Services lists the black soldier fly among feeder species in the category that does not require an import permit. That is a genuine advantage over some other feeders in the hobby. We ship black soldier fly larvae to Florida, as we do every feeder we sell except dubia roaches. Check the current state guidance yourself rather than relying on a shop page for it.
Where black soldier fly larvae are the wrong choice
They are the newer, more expensive, more interesting item, and the calcium story makes them easy to recommend. Several situations still point at the mealworm.
- You want to buy ahead. Mealworms cold-store. Black soldier fly larvae do not, so a large order is a countdown to pupation.
- Very small animals. The smallest mealworm grades reach sizes that are hard to match.
- The animal refuses them. Refusal is common enough that it is worth testing on a small quantity, and a refused premium feeder is worse value than a cheap one that gets eaten.
- Birds and poultry. Mealworms are the established item for birds and chickens.
- You are already dusting properly. If supplementation is under control, the main advantage of black soldier fly larvae is smaller than it sounds, and the rest of the comparison is a wash.
The reverse case is narrower but stronger: if you are feeding an animal whose supplementation you cannot control tightly, or you want a feeder that contributes calcium rather than diluting it, black soldier fly larvae do something no mealworm does.
Pick this one if…
- Pick black soldier fly larvae if you want a calcium-contributing feeder in the rotation and you can use them within a few weeks.
- Pick black soldier fly larvae if you are in Florida and want a feeder with no permit question attached.
- Pick black soldier fly larvae if low maintenance matters. They need no feeding and no cleaning while you hold them.
- Pick mealworms if you need small grades, want to hold stock cool, or feed birds as well as reptiles.
- Pick mealworms if the animal is dish-fed and already takes them. A feeder that gets eaten beats a feeder with a better label.
- Pick both if you are building a rotation. A review of feeder insect nutrition (Finke and Oonincx, 2023) recommends a varied diet over a single insect species.
If this is the first feeder decision you are making rather than the fifth, start at choosing a staple feeder insect, and read our shipping and live arrival terms before you order either one.