Mealworms are usually only occasional pests of the homeowner. They are considered scavengers and are among the largest insects that infest stored products. Most prefer to feed on decaying grain or milled cereals in damp, poor conditions. These insects are usually found in places not frequently disturbed such as dark corners, under sacks, in bins and where feed is stored. Indoors, they are usually found in basements or at grade level, sometimes infesting cornmeal, flour, cake mixes, cereals, meat scraps, dead insects, bran and litter from chicken houses. Mealworms have been found living under old carpeting and in straw-stuffed chairs in damp areas. Yellow mealworm larvae are known as “golden grubs” and make excellent fish bait and serve as food for animals in aquariums and zoological parks.
Young larvae are white, darkening with age. Larvae of yellow mealworms are honey-yellow, while dark mealworms are dark-brown.
Mealworms have a smooth, highly polished, shiny, elongate, hard, cylindrical (wormlike) body about 3-mm thick and up to 3.17-cm long at maturity. Beetles are robust, black and nearly 1.9-cm long, resembling many ground beetles in size, shape and colour.
Yellow mealworm adults are shiny, dark-brown or black, whereas dark mealworm adults are dull, pitchy black. Eggs are white, bean-shaped and about 1.27-mm long. Pupae are white at first, turning yellow, and are not enclosed in a case or cocoon.
Mealworm adults do not move as fast as ground beetles. Also, the hind leg of a mealworm adult has only four tarsal segments, while the hind leg of a ground beetle has five tarsal segments.
LIFE CYCLE & HABITS
Adult beetles are attracted to night-lights, are strong flyers, and are found in dark places. Each female lays about 275 eggs, which hatch into larvae in 4 to 14 days. Eggs are laid singly or in clusters during the spring over a period of 22 to 137 days. Larvae may spend up to two years in this stage depending on the environment. Individual larvae may wander far from the food source to pupate, making it difficult for homeowners to locate the source of infestation.
Most larvae mature by fall, overwinter and pupate in the spring, with the pupal stage lasting 7 to 24 days. Adults emerge in the spring and early summer, living two to three months.
Mealworms are not always considered pests since some people raise them for fish and animal food.
Use good night-light discipline to reduce beetle attraction and infestation indoors. Sanitation is the simplest means of controlling mealworms. Moist, dark, undisturbed areas containing damp grain or milled cereals should be eliminated. Litter remaining in old or abandoned poultry houses can create explosive infestations, bothering residents as far as 804 metres away. Poultry litter dumped into piles outdoors can result in mealworm migrations indoors. Waste grain products in a sanitary landfill may become a source of infestation.
Eggs and larvae of mealworms are sometimes accidentally eaten in cereals and breakfast foods. These infestations in foods and seeds can be eliminated by heating in a shallow pan in the oven at 54°C for 30 minutes or placed in a deep freeze at -17°C for seven days. However, seeds saved for planting may have the germination reduced after super heating or cooling. Sometimes simply collecting individual mealworms with a broom and dustpan and crushing before disposal is useful.
Pyrethrins alone or with a synergist has been used successfully for treatment of wooden floors or walls of empty granaries or warehouses. Insecticides labeled for mealworm control include pyrethrins (Pyrenone), resmethrin (Synthrin) and silica gel (Drione, PT 230 Tri-Die). Always read and carefully follow label directions and safety precautions.
This publication contains pesticide recommendations that are subject to change at any time. These recommendations are provided only as a guide. It is always the pesticide applicator’s responsibility, by law, to read and follow all current label directions for the specific pesticide being used. Due to constantly changing labels and product registration, some of the recommendations given in this writing may no longer be legal by the time you read them. If any information in these recommendations disagrees with the label, the recommendation must be disregarded. No endorsement is intended for products mentioned, nor is criticism meant for products not mentioned. The author, The Ohio State University and Ohio State University Extension assume no liability resulting from the use of these recommendations.
William F. Lyon
Keith L. Smith, Associate Vice President for Ag. Adm. and Director, OSU Extension.
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