Mealworms, Tenebrio molitor (biology – current uses – potential use in aquaculture)

Photo credit: Gerardo Ontiveros Lopez (Mexico)                                          Description and review: Abdel Rahman El Gamal

Introduction: Mealworms are the worm-like larval form of a very dark brown beetle, Tenebrio molitor, a species which belongs to the genus Tenebrio and the family Tenebrionidae.  The animal has other common names such as “Yellow mealworm”, “Mealworm beetle”, “Darkling beetle”, and “Darkening beetle”. The larval form is the usable form in pet’s keepers.

Distribution and habitats: Mealworms are found throughout most of the world in temperate regions, but primarily in the northern hemisphere. They thrive on grain and flour and are usually live near human habitation where their food sources are readily available. They prefer in dark, cool, and damp places like under decaying logs and leaves. When found inside homes, they are found where cereals and grains are stored.

Description: The larvae are light brown with darker brown stripes. The larvae grow to about 25 mm and can reach up to about 35 mm long. They have a segmented body, six legs (towards the front of the body) and two antennae. Mealworms have a somewhat hard outer shell

The pupa is white/cream with a large head and a pointed tail; it darkens as it grows. The mealworms adult –like all beetles- has a dark brown or black hard exoskeleton, six jointed legs, two antennae, and compound eyes. Its 12-25 mm long body is divided into three parts (the head, thorax, and abdomen). 

Feeding habits: Adults and larvae of “Mealworms” are scavengers that infest and feeds on stored grain products that is why it is considered a pest. Mealworms also eat decaying material, like dead insects, dead plants, and meat scraps. They are more active in damp or decay-promoting conditions. The insect gets their water requirements from the food they eat. Their affinity to eating grains makes them pests as they can get into and ruin stored grains of all types.

Mealworms often grow rapidly under favorable conditions. As they grow, they need to molt their outer shell, continue to grow until their new shell hardens. Mealworms go through series of molts and growth process during this stage of their life.

On the other hand, mealworms are eaten by many animals, including wild birds, rodents, spiders, lizards, and some other beetles. For self-defense, they spray noxious odor from scent glands.

Reproduction and life history: The mealworm is not a worm but the larval stage of the mealworm beetle which is also called the darkling beetle (Tenebrio molitor).

Like all holometabolic insects, the darkling beetle experiences complete metamorphosis and goes through four distinct life stages: egg, larva, pupa, and adult. The duration of each stage varies significantly due to environmental factors such as temperature, humidity and food. A female Darkling beetle lays about 270-500 small, white bean shaped eggs. Depending on water temperature, eggs hatch into tiny mealworm larvae in an average of about 10-12 days but this period could be much shorter while could extend to close 20 days or even beyond.

During the larval stage, and in order to grow, mealworms larvae undergo repeated molting/growth cycles. It is estimated that the number of molting may range from 10-20 times. By reaching the last molt and shedding its exoskeleton, the larvae lose its carapace before curling into its pupal form which is the third stage in the life cycle. The larval stage is the stage where the insect is a mealworm.

The length of pupal stage ranges widely depending on incubation temperature and can be as short as less than a week up to several weeks. During the pupation, the pupa color changes from a creamy white color to brown color. The pupa does not eat and seems inactive and lasts for a period that ranges from 1-3 weeks. During this period, the pupa transforms its organs and body into an adult. By the end of this phase, a white with a soft exoskeleton adult beetle emerges from the pupa. As the outer shell hardens, it will turn brown and then almost black. After about one to two weeks of adult life, beetles will begin to mate and reproduce. The adult lives for a few months which mean that the entire life cycle takes about a year.

Utilization of mealworm: The high protein content of mealworms promoted its use as a typical food source for amphibian, reptiles, aquarium fish, and wild birds, particularly during the nesting season, when birds are raising their young. They are commonly used in sport fishery as fishing bait such as for catching crappies or sunfish. In fact, there are various products of the mealworm prepared in variety of forms for human consumption; some of which are sorted as health care foods.

The steadily increase in the prices of commercial fish feeds, stimulated searching and looking for less expensive feed for production economics reasons. Research projects have been designed to investigate the possibility of incorporating the mealworms in fish feed such as Nile tilapia and African catfish feed.

The manufacturers of fish feed in their attempts of replacing expensive ingredients such as fish meal by less expensive comparative ingredients are expected to benefit from research outcomes and commercialize it. The incorporation of the mealworms in on-farm feed preparation is most likely justified especially the production of the mealworms could be achieved on small farm level.

Away from its possible use in pet or aquaculture systems, Tenebrio molitor is being used as a model organism in biology laboratories for biology, biochemistry, evolution, immunology and physiology. This is based on their relatively large size as well as the ease in their production and maintenance.

 References: Wikipedia, Encyclopedia of Life, Aqualand Fact Sheets, Entomophagy WIKI, mealworm store

Mealworms in aquaculture

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