Mealworms were once the staple of reptile and amphibian food in pet shops, but have fallen out of favour somewhat in recent years. This was mainly due to the discovery that (a) they were hard to digest, especially for smaller lizards, due to their tough exoskeleton, and (b) nutritionally they were not outstanding, the relation of "meat" to the outer shell not being very high.. What really frightened many keepers off, though, was the assertion that undigested mealworms swallowed whole could eventually burrow their way out of an animal, killing it in the process. This seemed especially probably in the case of smaller or soft-skinned lizards such as geckos and anoles. But before you dismiss mealworms, please read the advice below. If chosen for the right animals and if guidelines are followed, mealworms can make a good addition to the diet.
King mealworms, ironically enough, have actually become more popular in recent years. As their name implies they are scaled-up versions of mealworms, although the two types are not closely related. While king mealworms are only suitable for large or very aggressive lizards that will kill them before swallowing them, they are useful.
The designation of "worm" is actually misleading, since these creatures are not worms at all, but rather the larvae of beetles. There are actually several species of mealworm, but the one most commonly offered is Tenebrio molitor. Others occasionally offered, in least in the US, are Blapstinus moestus, apparently very similar to T. molitor, and the lesser mealworm, Alphitobius diaperinus, which is much smaller and a useful species to offer to small lizards such as anoles and geckos (see Zoffer, below). King mealworms likewise are the larvae of various Zophoba species, a larger beetle.
Both types of mealworm are sold in pet shops in plastic tubs that usually contain a layer of brown dusty material (all that's left of the food they've been nibbling on) and/or a layer of bran. In the case of the larger king mealworms, these are sometimes kept instead in a large plastic tank and the individual worms counted out into a plastic tub for the customer, together with a sprinkling of bran. The king mealworms, as you might expect, are more expensive, whereas a tub of mealworms will contain seemingly hundreds of these small thin brown larvae. Do not worry if the worms seem inactive: king mealworms in particular need warmer temperatures to be active. Dead worms are easily distinguished as dry, much darker carcasses that have often been chewed up by their tubmates.
It is important to realise that at the point of purchase the nutritional value of these creatures is actually not terribly great. This is because they have been kept in these conditions for some days, if not weeks, without food or water, and so lack much dietary goodness. If you feed them straight to your pet(s) as soon as you get the worms home, your animals will not derive much from them other than bulk. The way around this is gut-loading, a term much beloved of reptile- and amphibian-keepers. Basically it means you feed and water the insects for 12-24 hours before you feed them to your captives. This makes for a highly nutritious meal.
I have found the following method of keeping both mealworms and king mealwroms to be quite convenient and economical, leading to healthy creatures, fewer losses and in some cases breeding! First you need something to keep the worms in. An old and small aquarium with a fitting lid will do, but I have found it just as good and more portable to use the small plastic "Pet Pal" tanks that come in a variety of shapes and sizes with fitting lids that snap into place. They also usually have a small hatch in the top. Whatever receptacle you use, the method is basically the same. Sprinkle a layer of bran or oatmeal on the floor to the depth of an inch or so, then just tip the worms into the tank. You should see the king mealworms in particular eagerly burrow into the bran and disappear. You may or may not choose to lay a layer of newspaper over the top: I don't normally bother with this, but some people like to do it, and it does mean that king mealworms in particular will congregate beneath it, making it easier to grab a handful at feeding time. The lid should be clipped back on, as although I have never had any trouble with escapees, you should be aware that the tendency of mealworms to gnaw may extend to things like carpet. Also, some mealworms in particular will quickly go through the pupae stage and metamorphose into the adult small black beetle stage, which probably have the ability to fly, though I have never noticed them do so.
Mealworms and especially king mealworms do best when kept above 70 deg F, so I keep mine in the same room as my reptiles. King mealworms normally need temperatures of about 80-82 deg F before they start to breed, but mealworms may start at lower temperatures once there are a few adults in the colony. Zoffer recommends several layers of bran and burlap cloth in the tank to encourage breeding if you want to raise your own worms, and it certainly seems an easier process than trying to breed crickets.
Feeding mealworms is simplicity itself: they seem to thrive mainly on carrot or potato, although pieces of other fruit and vegetable can be experimented with. I prefer to buy fresh carrots for mine, placing one of each lengthwise in the tank on top of the bran. Not only does this attract the worms, but after a few hours or days I usually notice that the carrot, or what's left of it, has been pulled down into the bran. You should try to pull out anything that's left after 2-3 days, as otherwise the worms are left with a shrivelled and decaying stump that can't be too healthy for them. Another bonus is that the worms get their necessary moisture from their food, so water dishes are unnecessary and actually probably tend to encourage drowning or soggy bran instead.
King mealworms tend to prefer carrot or potato, but I have found mealworms are also partial to leaves of kale, greens and the like. It is a fascinating sight to watch worms swarm over this sort of food and a few hours and days later to see the holes left in it. Again, the greenery will dry out after a while, after which it should be replaced.
Something you should be aware of is that king mealworms can be very cannibalistic, especially in the absence of food. Therefore food should always be available to keep losses down, although I have found that inevitably some do occur.
As a rule I only offer the larvae stage of T. molitor, ie the wriggly brown worms, to those of my lizards that have very strong jaws and/or are on the large side. This means as a rule my plated lizards, Cunningham's skinks (both adults and juveniles) and savannah monitor, although he's not so good at being able to seize them. All of these lizards will usually crunch the prey up in their jaws before swallowing it, which ensures no mishaps. Also, lizards such as savannah monitors have very powerful digestive systems. I do not offer the leopard- or fat-tail geckos or our small cordylid these worms. The same applies even more to king mealworms.
An important exception is the pupae stage of the mealworm, when the worms shed their exoskeleton and become softer, yellow-white pupae that look a bit like a white chocolate trilobite with a body tapering to the rear. When you pick one of these pupae up you may notice the "tail" flicker and twitch, which is a good sign. These pupae can usually be fed safely to most lizards that are not too small, and as they contain little chitin they are easy to digest. However, obviously you should not overfeed them. I have found that most of our lizards will take these pupae quite eagerly. I do not offer king mealworm pupae for the simple reason that so few of them reach this stage that it's worth conserving them in the hope of getting some beetles you can breed from, though so far I have had no success. Similarly I rarely offer the adult beetles of either type, although they are quite edible for larger and tougher lizards.
Feeding mealworms and king mealworms to your animals is simplicity itself. Unlike crickets, I do not consider it worth cooling worms in the fridge as then they would hardly move at all, which would not attract their predators. Instead you can simply take a handful from the tub, shake them up in a plastic bag with some calcium powder supplement and then either tip them into the cage or offer them one or two at a time. Certainly I have found that this is a good way to hand-feed your pets and help to build a relationship with them, as most lizards will eagerly take the food from your fingers. Further, depending on the substrate used and how deep it is, it may be worth offering the worms individually, as if released all at once some may escape by burrowing into the substrate, which means at best that your pets have lost some food and at the very worst that the worm(s) may return to pester or even harm their erstwhile predators later on. This does not apply to pupae, which cannot move much at all, nor probably to the adult beetle stage.
It should go without saying, but captive animals should not be fed food items that are too large for them, nor too tough. A good rule of thumb I have found is that geckos and similar-sized lizards will only eat the pupae stage, while baby lizards of most species should not be fed mealworms larvae at all. Only offer king mealworms to medium- to large-sized lizards. You should not feed them to creatures which are habitually or wholly herbivorous such as chuckwallas, green iguanas or Mediterranean tortoises, as the digestive system of these creatures would probably be unable to cope with them. Read the notes above and if in doubt, use another food source
Very little maintenance of mealworm tanks is needed, other than to remove old and decaying food and perhaps with king mealworms to remove dried out carcasses. Bran and oatmeal seem to last indefinitely, but you may like to change it every now and again, especially if it gets damp.
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