Common Name: Dark Mealworm
Scientific Name: Tenebrio obscurus Fabricius
The common name describes the dark color of the larval stage and that it is a stored product pest. Dark mealworms are of moderate importance as stored products pests and are probably best known as fish bait and food for small pets such as turtles, reptiles, birds, small mammals, etc. They are of medical importance because they are often the species infesting the human gastrointestinal tract. Dark mealworms are worldwide in distribution and found throughout the United States.
Adults about 1/2-5/8″ (12-16 mm) long. Color dull (matte finish) black, sometimes very dark brown. Head with eyes notched on inside margin by a frontal ridge. Antenna 11-segmented, slightly clubbed, with attachment/insertion concealed from above. Pronotum with large and small punctures/pits intermixed, punctures very close to each other with many touching (confluent).
Elytra with punctures/pits in rows, area between rows mostly flat. Front leg tibia weakly expanded apically with blunt or rounded edge dorsally on apical half. Tarsi 5-5-4. Mature larvae up to about 1 1/4″ (32 mm) long; very smooth, cylindrical, hard bodied. Color yellowish brown in middle changing to blackish brown towards both ends. Terga (dorsal plates) of 2nd and 3rd thoracic and 1st abdominal segments each with a distinct transverse raised line (carina) near its front.
Last abdominal tergum (dorsal plate) with 2 terminal upturned acute processes (urogomphi), with 2 short stout ventral hair (seta) and tibiotarsus (fused tibia and tarsus terminating with a claw) with 4-5 ventral hairs (setae).
(1) Yellow mealworm (Tenebrio molitor) with pronotal punctures/pits not touching, front leg tibia moderately expanded apically with a sharp edge dorsally on apical half, and body surface weakly shining.
(2) Meatus tenebrioides has pronotum with very large punctures laterally and small punctures overall.
(3) Lesser mealworm (Alphitobius diaperinus) much smaller (1/8-1/4″/3-7 mm long), dorsum shiny and front leg tibia broadly expanded apically.
(4) Flour beetles (Tribolium spp.) have elytra with a fine distinct longitudinal ridge between each lateral row of punctures and much smaller, usually less than 1/4″/7 mm long.
The adult female lays beanshaped, white, sticky eggs which quickly become covered with surrounding food particles. Eggs are laid singly or in cluster in food material over a period of 22-137 days, with an average of 463 (range up to 500) eggs being laid per female. At 65-68F/18-20C, eggs hatch in 10-12 days. Larvae develop slowly, usually going through 14-15 molts (range 9-20) in about 4-18 months.
Mature larvae crawl to near the food surface to pupate and the pupal stage lasts about 20 days. Developmental time (egg to adult) usually requires 280-630 days. At 77F/25C, larvae developmental time can be shortened to 6-8 months and the pupal stage to 9 days. There is usually 1 generation per year but some individuals may require up to 2 years to complete their development. Adults usually live 2-3 months.
This species is of medical importance because eggs and/or larvae ingested with cereals or breakfast foods can cause gastrointestinal discomfort. Whether eggs and/or larvae are ingested, live larvae may be passed in the feces.
Dark mealworms are usually found breeding in grain refuse and debris, and typically indicate a lack of proper sanitation. They prefer a dark and damp environment. They have been found in neglected corners with accumulated grain, under bags of grain in warehouses and feed stores, in spillage around grain bins, in the litter of poultry houses where grain is mixed with droppings, and in other material of animal origin, including dead insects.
Follow the standard control procedures for stored product pests. Pay particular attention to dark and damp situations with grain spillage or accumulation.