Mealworms are the larvae of two species of darkling beetles of the Tenebrionidae family, the yellow mealworm beetle (Tenebrio molitor Linnaeus, 1758), and the smaller and less common dark or mini mealworm beetle (Tenebrio obscurus Fabricius, 1792). Mealworm beetles are indigenous to Europe and are now distributed worldwide. Tenebrio molitor is a pest of grain, flour and food stores, but often not of much importance since populations are quite small (Ramos-Elorduy et al., 2002). Mealworms are easy to breed and feed, and have a valuable protein profile. For these reasons, they are produced industrially as feed for pets and zoo animals, including birds, reptiles, small mammals, batrachians and fish. They are usually fed live, but they are also sold canned, dried, or in powder form (Aguilar-Miranda et al., 2002; Hardouin et al., 2003; Veldkamp et al., 2012).
The life cycle of Tenebrio molitor is of variable length, from 280 to 630 days. Larvae hatch after 10-12 days (at 18-20°C) and become mature after a variable number of stages (8 to 20), typically after 3-4 months (at ambient temperature) but the larva stage can last up to 18 months. The mature larva is of a light yellow-brown colour, 20 to 32 mm long, and weighs 130 to 160 mg. Commercial mealworm producers sometimes include a juvenile hormone into the feed to prevent the larvae from molting into adults, resulting in "giant" mealworms that can achieve a length of 2 cm or more, and weigh more than 300 mg (Finke, 2002). The pupal stage lasts 7-9 days at 25°C and up to 20 days at lower temperatures. The adult Tenebrio molitor lives for 2 to 3 months. The life cycle of Tenebrio obscurus is shorter, particularly in the larval stage (Hill, 2002; Hardouin et al., 2003).
Feeding and nutrition
Mealworms are omnivorous and can eat all kinds of plant material as well as animal products such as meat and feathers (Ramos-Elorduy et al., 2002). They are typically fed on cereal bran or flour (wheat, oats, maize) supplemented with fresh fruits and vegetables (carrots, potatoes, lettuce) for moisture together with protein sources such as soybean flour, skimmed milk powder or yeast (Aguilar-Miranda et al., 2002; Hardouin et al., 2003). The diet should be balanced to contain about 20% protein (DM basis) (Ramos-Elorduy et al., 2002).
Mealworms are able to utilize the small amounts of water contained in dry feeds but the productivity of water-deprived mealworms is low (one generation per year). It is preferable to provide them with a source of water for better productivity (up to 6 generations per year) and to prevent cannibalism. Relative humidity is linked positively with fertility and adult activity. It is necessary to monitor fresh feeds as they may turn mouldy (Hardouin et al., 2003).