Feeder Insect Nutrition: What the Numbers Actually Say

Max · Reptile keeper · Last reviewed October 17, 2026

Not one feeder insect in the Merck Veterinary Manual's composition table reaches a 1:1 calcium to phosphorus ratio. That single fact explains supplementation, gut loading, and most metabolic bone disease in captive insectivores.

The composition table

These are the Merck Veterinary Manual's figures, reproduced as published. Values are on a dry-matter basis; protein and fat percentages look high for that reason. The last column is what matters most for a growing or egg-laying animal.

Food item	Dry matter %	Protein %	Fat %	Calcium %	Phosphorus %	Ca:P
Mealworms (Tenebrio)	42.2	52.8	35	0.06	0.53	0.11
Superworms (Zophobas)	40.6	17.4	17.9	0.01	0.02	0.43
Locusts	31.2	61.7	19.4	0.1	0.75	0.13
Crickets	38.2	55.3	30.2	0.23	0.74	0.31
Earthworms	22	49.9	5.8	0.59	0.85	0.69
Wax worms	38.3	15.5	22.2	0.03	0.22	0.13
Chicken muscle	25.6	20.5	4.3	0.01	0.2	0.05
Egg, whole	25.2	12.3	10.9	0.05	0.22	0.02
Mice, 1–2 days old	—	—	—	1.6	1.8	0.88
Mice, adult	—	19.86	8.81	0.84	0.61	1.37

— indicates a value Merck lists as not available. Source: Merck Veterinary Manual, read October 17, 2026.

What the ratio column means

Calcium and phosphorus compete. When a diet carries more phosphorus than calcium, the animal pulls calcium from its own skeleton to balance the difference, and over months that becomes metabolic bone disease. A ratio below 1.0 in that last column means the food is working against the animal's bones unless something is added.

Look down the column and the pattern is stark. Mealworms 0.11. Wax worms 0.13. Locusts 0.13. Crickets 0.31. Even earthworms, the best invertebrate on the list, reach only 0.69. The two entries that clear 1:1 are both rodents, and adult mice at 1.37 are the only item approaching the preferred 2:1.

Gut loading beats dusting, and Merck says so directly

Most keepers dust. Merck's assessment of dusting on its own is blunt: it is "doubtful to add enough calcium to meet most animals' nutritional needs." The powder sits on the outside of the insect, some of it falls off in the enclosure, and the animal may groom the rest away.

Gut loading changes the insect itself. Merck describes feeding crickets or mealworm larvae a balanced mixture containing 12% calcium as calcium carbonate for three days before use, which raises the insect's calcium from a trace to roughly 0.8 to 0.9% and produces a ratio near 1.2:1. A separate Merck figure gives 72 hours on a supplement of at least 8 to 10% calcium. Either way the principle is the same: feed the feeder first, then dust immediately before offering.

Why every chart you find is slightly different

An insect's nutritional content is not a fixed property of the species. It reflects what that insect was raised on and how recently it ate. A cricket shipped dry and starved for two days is a different food from one that spent 72 hours on a calcium diet. This is the honest limitation of every feeder chart published anywhere, this one included. Use it to compare feeders against each other, not as a guaranteed label.

Common questions

What is the right calcium to phosphorus ratio for reptiles?

The Merck Veterinary Manual states the ratio of food offered to reptiles should be at least 1:1, with 2:1 preferred. Every invertebrate in Merck's own composition table falls below 1:1, which is the entire reason calcium supplementation exists.

Which feeder insect has the best calcium to phosphorus ratio?

Of the invertebrates in Merck's table, earthworms are highest at 0.69, followed by crickets at 0.31 and superworms at 0.43. All three are still inverted, meaning phosphorus exceeds calcium. Only adult mice, at 1.37, clear the 1:1 mark.

Is dusting insects with calcium enough?

On its own, probably not. Merck states plainly that dusting insects with a calcium-phosphorus powder is "doubtful to add enough calcium to meet most animals' nutritional needs." The recommended approach is to feed the insects a high-calcium diet first and then dust immediately before offering.

What is gut loading and how long does it take?

Gut loading means feeding the insect a calcium-rich diet before it is fed to your reptile. Merck describes feeding crickets or mealworm larvae a mixture containing 12% calcium as calcium carbonate for three days before use, which raises insect calcium to roughly 0.8 to 0.9% and produces a ratio near 1.2:1. A separate figure gives 72 hours on an 8 to 10% calcium supplement.

Are mealworms bad for reptiles?

They are not poison, but the numbers explain the reputation. Merck puts mealworms at 35% fat on a dry-matter basis with a Ca:P ratio of 0.11, among the worst in the table on both counts. As an occasional item they are fine; as a staple they combine high fat with very little calcium.

Why do published feeder nutrition charts disagree with each other?

Because an insect's composition depends on what it was fed and when it was last fed. A gut-loaded cricket and a starved one are not the same food. Treat any chart, including this one, as a snapshot of insects raised on one particular diet rather than a fixed property of the species.

Sources

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