First published in Herpetoculture House Digital Magazine vol. 4 issue 2
Two of the most common insects used to feed various species of reptiles, amphibians, and even arachnids are mealworms and superworms. Both types of “worms” are the larvae of different species of darkling beetles, mealworms and superworms are the species Tenebrio molitor and Zophobos morio, respectively. Referring to them as “worms” is a bit of a misnomer as they have a hard shell and legs. Mealworms and superworms like all beetles go through four life stages from egg to larvae to pupae, and finally into mature beetles. Mealworms, in particular, are some of the simplest feeders to keep and breed. They are relished by reptiles, birds, fish, and even some adventurous humans interested in culinary experimentation.
Mealworms are well suited to feeding small lizards and juvenile arachnids as they are typically only two and a half centimeters in length and the adult beetles measure between 1.25 and 1.8 centimeters. Mealworms thrive best in temperatures ranging from the mid to low 70's to a high of 80 degrees with a relative humidity around 70%. Mealworms can be easily kept in smooth sided plastic containers, such as shoe boxes or sweater boxes, with either drilled air holes, a screened section cut out of the top, or no top at all since mealworms and darkling beetles cannot climb a smooth surface or fly. Some people also keep mealworms in kritter keepers or ten gallon glass tanks. It is best to prepare at least two of these containers, one for the larvae and one for the beetles, although a third and fourth container may also be necessary for holding pupae or when changing substrate. Adequate humidity levels can be maintained by placing a small bowl of water or a damp sponge inside a plastic sandwich bag in the containers of mealworms and beetles, if necessary. They can be kept on a variety of substrates including rolled oats, wheat bran, corn meal, a mixture of different types of grain, non-medicated chicken feed, or a combination of the above in a layer that is about two to three inches deep. Mealworms can also be supplemented by adding brewer's yeast, powdered milk, or bone meal to the substrate. Mealworms will also need a source of moisture in the form of hard fruits and vegetables such as sliced potatoes, carrots, apples, or any cooking scraps that are not likely to spoil or grow moldy too quickly. Vegetables and fruits should be replaced as needed but at least once a week. As the mealworms feed on the bedding material they will shed their exoskeletons as they grow and eventually transform into lighter colored pupae which are usually found above the layer of substrate. Mealworms typically molt between nine and twenty times over a period of about ten weeks at optimal conditions before becoming pupae. Pupae should be separated from the mealworms and placed in a smaller container until they metamorphose into beetles within six to eighteen days and then removed to the beetle container. Mealworms require very little care and cleaning. While some people will periodically sift the mealworms and change the old substrate, I personally, find it less wasteful to simply remove the dead individuals and old food manually which only takes a few minutes if done multiple times a week unless the frass (waste products) build up too much or a foul odor is detected in which case the substrate will need to be changed. The eggs and very young mealworms are almost microscopic in size and nearly impossible to sort out of the substrate so it is advisable when changing substrate to save the waste for a few weeks until any remaining eggs hatch and the small mealworms can be sorted out. One important note is that while mealworms can be stored in a refrigerator to prolong the larval stage for several months, superworms will die when exposed to such low temperatures.
Superworms also known as kingworms are quite a bit larger than mealworms and measure between five and six centimeters in length and enjoy burrowing in three to four inches of the same kind of substrate used for mealworms. They have less chitin content than mealworms making them a more digestible meal for reptiles. They are a good sized meal for larger lizards and adult arachnids. Superworms are also capable of biting humans which typically feels like a sharp but harmless pinch. They can be kept in conditions similar to mealworms with a few key differences related to pupation and breeding. Superworms require slightly higher temperatures than mealworms and fare best in temperatures ranging from the high 70's to mid 80 degree range. They will also require a consistent source of moisture in the form of hard fruits and vegetables to prevent cannibalism. Superworms that are between eight to ten weeks old are much more likely to pupate when separated individually from the rest of the colony. Craft boxes with individual compartments or old film canisters are some of the easiest ways to separate superworms. Superworms that are ready to pupate will become dormant and lay in a curled up position before going into the pupal stage, which lasts from two to four weeks at optimal temperatures.
Breeding darkling beetles
Newly formed darkling beetles of both species will be whitish in appearance darkening to red and then black once their new exoskeletons harden. Darkling beetles of either species will mate readily. A male will chase a female until he is able to mount the female and inject a sperm packet using his aedeagus, which is the male reproductive organ. The female beetle will typically lay around three hundred eggs a few days after mating occurs. The eggs will hatch into tiny larvae within four to nineteen days for T. molitor and seven to fourteen days for Z. morio. The cycle of mating and egg laying will begin nine to twenty days after metamorphosis and continue for eight to twelve weeks for T. molitor and up to sixteen weeks for Z. morio at which point the adult beetles will die. Once I see a number of visible tiny worms in the beetle container, I will sift the substrate, worms, and eggs back into the worm bins and give the beetles fresh substrate.
Breeding mealworms and superworms can be a simple and cost effective process for anyone that has a large number of insectivorous animals to feed.