Yellow Mealworm (Tenebrio molitor)
Dark Mealworm (Tenebrio obscurus)
The adult is a dark brown or black beetle about 12mm long. Its thorax is finely punctured, and its wing covers are longitudinally striated or grooved. Both yellow mealworms and dark mealworms have well-developed wings and are attracted to light. Yellow mealworm adults are shiny, dark-brown or black, whereas dark mealworm adults are dull, pitchy black.
Mealworms have a smooth, highly polished, shiny, elongate, hard, cylindrical (wormlike) body about 3mm thick and up to 32mm long at maturity. Young larvae are white, darkening with age. Larvae of yellow mealworms are honey-yellow, while dark mealworms are dark-brown.
Mealworms are among some of the largest insect pests of stored products. Their common names are derived from the color of the wireworm-like larvae. Both yellow and dark mealworms are in the genus Tenebrio, meaning "darkness," owing to the nocturnal habits of the larvae. Believed to be of European descent, both insects are cosmopolitan, with the yellow mealworm more prevalent in the cooler northern states.
The larva can do serious injury to whole grains in storage for long periods. The majority of the damage is largely limited to contamination of the products by the worms and their waste products.