Mealworm, the Perfect Fish Food?
Text & Images by Paul Barrow

Mealworm (Tenebrio molitor) are versatile; they not only make excellent food for a variety of pet but also act as stunt doubles for maggots in many TV detective series. Unlike maggots, mealworm are clean, dry, free from pathogens and only eat the best bran flakes. Their healthy lifestyle makes them highly nutritious and safe, everything needed to maintain healthy fish. Mealworm are the larvae of the Mealworm Beetle, Tenebrio molitor , which now completes the whole of its lifecycle eating the milled flour made by us. They are so successful in their pilfering ways that they have a marked outcome on the economies of some businesses. Like most of the order Coleoptera (Beetles), they go through a 4-stage lifecycle: egg, larvae, pupae and beetle. 
The Eggs. Don't bother looking you won't find these minute objects. The Larvae Two Larvae showing the hard exoskeleton
The larvae of these beetles grow to just over 2.5cms, and resemble a hard-shelled caterpillar. A larva showing the two rows of legs at the front
They have a distinct, dark head and a long brown body with two rows of 3 legs at the front. Their lifecycle from egg to sexually mature beetle is around 60-days, however the different stages in the lifecycle can be highly variable, potentially increasing this time up to 5 months. By far the longest of these stages is, fortunately for us, the larval stage, which can last from 2 weeks to a few months. The Pupae The mealworm pupae showing a partial transformation towards the beetle stage.
These are strange, pale squat things that appear dead until touched when their tails rotate vigorously.
The Beetles The fully matured beetle (arrow 10mm approx)
They are small, less then 2cms, matt black creatures very much like the Ground Beetles found in the garden but, unlike their garden cousins, they do not bite and are not aggressive. Culturing Culturing these creatures could not be simpler, a large plastic or glass container about 25 x 20 x 20cms, filled nearly to the top with bran or oatmeal and covered with a fine mesh net will produce a good supply of mealworm for a year or more. Add to this a tub of shop bought mealworm, first removing any dead ones. Leave the container in a warm, dry place for nature to take affect, and within less than a week the first beetles will appear. Newly emerged beetles are very pale and go through a gradually darkening brown stage until fully black. Once mature they will start mating, taking as little as 4 days for their eggs to hatch into minute larvae. The larvae grow rapidly and need to shed their outer coating 10 or more times to maintain this growth. During the last split, they turn into pupae instead of larvae. After a week or so, the pupae splits to reveal another pale beetle and cycle starts again. A fully functioning community of mealworm will produce a variety of sizes, and can be used for most sizes of fish once sorted. When the worms split form their shells they are soft, white and suitable for fish with the tenderest of tastes. Don't feed the beetles or pupae, as they will be needed to produce the next generation. You will not get your mealworm to grow as big as those in the shops because their growth rate was artificially altered to increase size. Regularly sieving the bran results in a whole range of sizes. At their smallest (less than 1mm in diameter), mealworm are easily taken by fish as small as Neon Tetras. As their size increases, they can be fed to medium sized community fish. It does not seem economical to feed very larger fish as a couple of these could deal with a whole shop bought container in one go. Mealworm tend to float because of their dry environment, so if you want to feed them to bottom dwellers such as catfish, place an apple or potato slice on top of the bran for a few days, the extra moisture makes the worms sink.










Giant Mealworm (Zophobas morio)
A giant mealworm larva (Zophobas morio) compared to the larva of Tenebrio molitor
This is a species of Darkling Beetle group, which also includes the Mealworm Beetle. The larvae of this beetle, known as the Giant Mealworm or Superworm (5-6cms), were introduce to the hobby from South America in the mid 1980s. They need to be maintained at a much narrow temperature range (21-27oC) than the common mealworm. They can be kept in a variety of substrates but bran seems to be the easiest if they are not to be bred. In fact, they are not easy to cultivate because they can stay in their larval state for a very long time. Unlike mealworms, giant mealworms need moisture because they will die or cannibalise their brethren. A simple answer to this problem is to include pieces of apple, potato or greens. They are highly nutritious, with a much lower chitin (hard exoskeleton) content than ordinary mealworms making them easier to digests. Mealworm may be a good fish food but they must be used in conjunction with a balance diet which should include staple flake or pellets. Don't worry about the levels of chitin being fed because I have fed fish solely on mealworm for a week with no ill effects. In fact, chitin is probably beneficial because of the high calcium levels.