Quick answer: calciworms, the larvae of the black soldier fly, carry more calcium than phosphorus. Mealworms carry far more phosphorus than calcium. For animals that need calcium, which covers most reptiles, turtles, birds and shell-building invertebrates, calciworms are the better everyday insect, while mealworms are fine as an occasional extra if you balance them. Neither is a complete diet, dried insects must not be fed to chickens in Great Britain, and hedgehog rescues advise against both.
What they are
Calciworms are the larvae of the black soldier fly, Hermetia illucens, sold as calci worms, soldier grubs or black soldier fly larvae. Mealworms are the larvae of a darkling beetle, Tenebrio molitor. Both are sold live and dried, and both are eaten readily by everything from bearded dragons to robins.
The number that matters: calcium to phosphorus
Animals with bony skeletons or shells need calcium, and they use it alongside phosphorus. A diet with much more phosphorus than calcium works against them: the body draws calcium from its own reserves to balance it, and over time that shows up as weak bones, poor shells and, in reptiles, metabolic bone disease.
This is where the two insects part company. Published figures put mealworms at somewhere between one part calcium to seven parts phosphorus and one to fifteen. Calciworms come out the other way round, at roughly one and a half parts calcium to one of phosphorus. So a handful of calciworms adds calcium to a diet, while a handful of mealworms pulls it the other way.
Which to feed, animal by animal
Reptiles
Most insect-eating reptiles prefer live food, and live mealworms can be gut loaded and dusted with calcium, which goes a long way to fixing their balance. Where a reptile will take dried insects, calciworms are the better choice because the calcium is already in them; bearded dragons are among the lizards that often accept them, softened in warm water. Your reptile's care sheet should still set how often you dust with calcium.
Turtles and terrapins
Both make good occasional treats, but calciworms bring calcium for the shell, and turtles take them readily from the water. Keep either as an extra on top of a complete turtle pellet.
Garden birds
The RSPB suggests mealworms as a good food for garden birds, especially in the breeding season, and calciworms are used the same way. Soak dried ones in warm water before you put them out in spring and summer, offer small amounts, and don't leave food out overnight where hedgehogs can reach it.
Fish
Larger aquarium and pond fish take both as an occasional protein extra, broken up for smaller mouths.
Isopods and millipedes
For invertebrates that build and harden a new shell at every moult, calciworms give protein and calcium in the same mouthful. Crush or break them for small species and clear what is left after a day or two. Our isopod calcium guide covers the other calcium sources worth keeping in, and feeding insects to your isopods covers how often protein should go in.
The two don'ts
Chickens
Dried mealworms used to be a favourite treat for backyard hens, but under Defra's rules dried insects, calciworms included, cannot be fed to chickens or other poultry in Great Britain, even pets. Live insects your hens find for themselves are a different matter.
Hedgehogs
Hedgehog rescues advise against mealworms because the high phosphorus draws calcium from the bones, and many advise against calciworms and other dried insects too. If you want to feed hedgehogs, meaty cat or dog food is the usual recommendation.
Dried or live
Live insects can be gut loaded before feeding, which improves what they carry, and moving prey suits animals that hunt by sight. Dried insects store for months, need no feeding or housing, and are easy to portion, but nothing can be loaded into them after drying, so what is in the insect is what your animal gets. That is the strongest argument for calciworms in dried form: the calcium is built in.
Our dried calciworms are whole larvae with nothing added, and you will find them with the rest of our dried foods on foods for reptiles, turtles and fish.
Frequently asked questions
Are calci worms better than mealworms?
For calcium, yes. Calciworms carry more calcium than phosphorus and mealworms carry far more phosphorus than calcium, so calciworms are the better choice for reptiles, turtles, birds and invertebrates that need calcium. Neither is a complete diet.
Can you feed calciworms instead of mealworms?
For most animals, yes, and it is usually an improvement. Soften dried ones in warm water for birds in the breeding season and for reptiles that take dried food.
Why are mealworms bad for hedgehogs?
They are high in phosphorus and low in calcium, and hedgehogs fed on them can lose calcium from their bones. Rescues recommend meaty cat or dog food instead, and many advise against calciworms too.
Can chickens eat dried calci worms?
Not in Great Britain. Defra's rules don't allow dried insects, including calciworms and mealworms, to be fed to chickens or other poultry, even pets.
Do calciworms need soaking?
Not always. Soak them for garden birds in spring and summer, and soften them for reptiles and small turtles; isopods and millipedes take them dry or lightly crushed.
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