Mealworm beetles are large beetles, two species of which are the largest of the stored product beetles at 5/8 inch to 4/5 inch (14 to 20mm) in length. They are dark brown or black in colour and, to an untrained eye, may resemble ground beetles in appearance. Mealworm beetles are uniformly oval in shape and flattened and the antenna is composed of 11 segments. The tarsi of the mealworm beetle’s front and middle legs have five segments, but the tarsi of the hind leg has only four segments. Ground beetles have five segments in the tarsus of the front, middle and rear legs, an important identifying character.
Mealworm beetles are usually found within grain elevators, grain storage facilities, farms and feed stores. Due to the large amounts of grain processed in grain storage and processing facilities, grain spillage is common and may be allowed to accumulate in conveyor tunnels, basements and similar undisturbed areas. Such grain then becomes moist, begins to mold and develops into an attractive food source for mealworms.
Other than out-of-condition grain, mealworm have been found infesting cereals, bread, crackers, meat scraps and mill scraps.
The yellow mealworm beetle, Tenebrio molitor, and the dark mealworm beetle, T. obscurus, are the large varieties and can easily be confused with one another. The yellow mealworm adult is colored a shiny dark brown or black while the dark mealworm beetle is a dull pitchy black color. The larvae of these two beetles are long, thin, hard, and wiry and grow up to 1 ¼ inch (30 mm) in length. Those of the yellow mealworm beetle are a honey-yellow color while the larvae of the dark mealworm beetle is brown in color, being darker towards each end.
The adult female lays several hundred bean-shaped eggs that are coated with a secretion that sticks to flour and grain dust. The eggs hatch within two weeks into thin white larvae that soon after begin acquiring their yellow or dark brown coloration. The larvae undergo an unusually high number of molts (14 to 15) over a period lasting several months and up to more than 600 days. Pupation occurs close to the surface of the food medium and pupa are easily found due to their large size. Mealworm beetles are strong fliers and are attracted to bright lights. Eggs laid in cereals or breakfast foods can be consumed by people and the larvae actually hatch and survive in the stomach and intestines. Such an occurrence creates gastrointestinal pain and distress.
Yellow and dark mealworm beetles prefer moldy, moist and out-of-condition grain products so inspection should focus on identifying sites where these food sources could be allowed to accumulate. Move grain about with a screwdriver or a knife and look for larvae, pupae and adult beetles. All should be easy to see due to their large size.
When infestations of yellow or dark mealworm beetles are found, removal of all infested food product should be completed. Treatments of cracks in those areas with a residual insecticide may be needed in case adults or larvae are harboring there.
Controlling lesser mealworm beetles in poultry houses requires a plan of regular sanitation and treatments with residual insecticides. When the chickens or turkeys are removed from a broiler house, the building is usually scheduled for cleaning. All old litter must be removed and the floor thoroughly washed. A general treatment with an appropriately labeled residual insecticide can then be made to the floors and walls of the building.