To quote an old adage “you are what you eat”, or perhaps more correctly; the health of your reptile is largely dependent on the diet you feed it. Although I suspect many people will tell you a varied diet containing a mixture of commercially available insects is best for your reptile, the truth is that proper nutrition goes much further than this. Tables 1 and 2 show nutritional values typical of the most common feeder insects available to herpetoculture today.

Table 1 Proximate nutrient composition of commercial invertebrate prey.

Species	Latin Name	Water %	Crude Fat (%DM)	Total Nitrogen (%DM)	Crude Protein (%DM)	ADF-N (%DM)	NDF (%DM)	Ash (%DM)
Adult Cricket	Acheta domesticus	62-73	19-44	10	40-68	0.7	19	2.7-5.1
Juvenile Cricket	Acheta domesticus	67	10	9	40-50	0.6	16	9.1
Mealworm	Tenebrio molitor	56-66	31-60	8-9	35-55	14	4	3.0-7.0
Superworm	Zophobas morio	57-59	41-44	6.9	40-50	0.4	13	2.9-3.5
Waxworm	Galleria mellonella	62-63	51-73	5.5	21-41	0.4	12	2.7-3.3
Silkworm	Bombyx mori	61-80	4-21	8.7-10	65	NA	NA	5.2
Fruit Fly	Drosophila melonogastor	67	18	9.0	NA	NA	NA	5.2

 

Acid-detergent fibre-nitrogen (ADF-N) is used as a measure of nitrogen in Chitin (a non-protein nitrogen), and neutral detergent fibre (NDF) is used as a measure of complex carbohydrate. DM, Dry mass. NA, not available.

There is one thing, which is immediately apparent upon looking at tables 1 and 2. This is that there are clearly differences in composition percentages between species, or to put it simply, components such as fat or water vary depending on insect species. This is perhaps not surprising, and may go some way to explaining why a “varied diet” is often suggested as a way of maintaining healthy reptiles. But it goes further than this, if you take a look at any of the absolute values in the tables, calcium percentage (Ca %) in Adult crickets for example (Table 2), you will notice that this is a relatively low figure, and to achieve the recommended daily calcium intake for any given animal on simply commercially purchased adult crickets, you would have to feed a very large number of individuals. This method would not be without drawbacks however, as the same species has a relatively high fat percentage (Table 1).

Table 2 Mineral concentrations (dry matter basis) of commercial invertebrate prey.

Species	Latin Name	Ca %	Mg%	P%	Cu (mg/kg)	Fe (mg/kg)	Mn (mg/kg)	Zn (mg/kg)
Adult Cricket	Acheta domesticus	0.1-0.2	0.08	0.8-1.4	8.5	112	30	186
Juvenile Cricket	Acheta domesticus	0.1-1.3	0.16	0.8	9.6	197	53	159
Mealworm	Tenebrio molitor	0.04-0.12	0.28	0.9-1.4	18	40	6.8	131
Superworm	Zophobas morio	0.03-0.12	0.18	0.6-0.8	14	50	1.5	88
Waxworm	Galleria mellonella	0.06-0.07	0.09	0.6-1.2	3.1	77	3.3	79
Silkworm	Bombyx mori	0.21	0.24	0.54	NA	NA	NA	NA
Fruit Fly	Drosophila melonogastor	0.14	0.13	1.1	8.7	454	16	147

You can make a number of these comparisons, but I would suggest that even maintaining animals on the most calcium rich feeder species would not afford your animals all the nutrients they require for a healthy life.

Table 2 also provides insight into the Calcium: Phosphate (Ca:P) ratio of each species. This is a very important factor to consider when assessing the diet of your reptile as maintenance of their natural Ca:P  ratio, 2:1 is essential. What does this mean in simple terms, well, the skeleton of an individual is made up of both calcium and phosphate, and to maintain this it requires both of these elements. However if you have an imbalance towards phosphate, this can lead to calcium ion leaching. This can lead to serious medical conditions including metabolic bone disease (MBD).

So from dissecting various components of the diet of many reptiles in captivity it becomes clear that not only are their nutritional needs not being met, but that without meeting them, their health can be affected. In order to remedy this, and increase nutritional content in all feeder species you must feed them a balanced diet. I’m sure all of you will be aware of ‘Gut-loading’ as it commonly referred too, but I cannot stress enough its importance for maintaining healthy captive reptiles. A question often posed at this juncture is “well who gut-loads insects in the wild?” The answer is that insects will eat a varied diet in the wild, of which their prey will also have eaten a varied diet. This means stored nutrient contents of wild insects will be much higher than that of commercially available insects, which are typically raised on bran.

Ok, so what constitutes a good varied diet for feeder insects then, well just that, variety. These insects will typically eat a huge range of vegetables and leaves. So it is a good idea to feed root vegetables, fruit, and a dark leaves 24-48 hours before you intend to feed them to your reptile. Not only does this vastly increase the nutritional value of the insect, but it will also allow it to live longer while stored, which should cut down on the number of boxes you need to buy!

Is this enough? Is properly gut-loading your insects enough to give your reptile all the nutrients it needs to live a healthy life? The answer to this is unfortunately no. Although this will increase the concentration of a variety of nutrients in their diet, it is pretty unlikely that you will achieve a calcium: phosphate ration of 2:1 this way. For this reason it is also highly recommended that you supplement the insects with a separate source of calcium. ‘Dusting’ the feeder species with a source of calcium immediately before it is eaten is a good way to dramatically alter the Ca:P ratio in favour of the calcium. This should allow for healthy bone growth and maintenance.

Calcium metabolism however, requires another component, D3. This is typically synthesised from UVB light. This is fine if your animal is diurnal, and has a UVB light source. However if you have a nocturnal animal, without a ready source of vitamin D3 then you should consider using a multivitamin powder as part of your supplement schedule.  There are a variety of species out there, and so it is not practical to give a general supplementation schedule which will suit all insectivorous reptiles, however a small amount of searching on the internet should allow you to find a schedule for your chosen species.

Finally, one other interesting point that can be taken from Table 1 is the column labelled ADF-N (%DM). This column is a proxy measurement of Chitin content in the exoskeleton of invertebrates. As you can see, all invertebrates have it in varying concentrations, however mealworms (Tenebrio molitor) have it in substantially higher volumes than most other species. But is this a problem? The simple answer to this is no, chitin is broken down in one of two ways, either by intestinal symbionts or by the presence chitinolytic enzymes. So why then are there frequent reports of undigested mealworms being passed in the feces of many species? Well this could be due to a number of things, the most likely of which is that another aspect of their husbandry is not been met. Temperature is would be a good example of this. As reptiles are exothermic, and so require external heat for bodily functions, if this heat is not present, or not at the required temperature then proper digestion of food items becomes difficult. Further to this, in many of the cases of this phenomenon I have heard of, it is only the empty chitinous shell of the mealworm that is passed. This is not necessarily a bad thing, as the main nutrient containing part of the body in contained within this shell, and so if it is empty, that means that the likelihood is, that this part of the mealworm has been digested.