Lesser Mealworm
Alphitobius diapernis (Panzer), Tenebrionidae, COLEOPTERA

DESCRIPTION

Adult -- Lesser mealworm adults are shiny black, somewhat flattened darkling beetles with an oval outline (6 mm long). The head and thorax are densely covered with tiny depressions. The wing covers have tiny depressions arranged in more or less parallel rows. Newly molted adults are reddish-brown and slowly turn black.

Egg -- Lesser mealworm eggs are slender with rounded ends (1.5 mm long). The eggs are creamy white but darken with age.

Larva -- Lesser mealworms are slender, segmented, worm-like insects with three pairs of tiny legs on the thorax and one abdominal proleg at the rear. Lesser mealworms grow to 7.5 mm long.

Pupa -- Lesser mealworm pupae are somewhat worm-like in appearance and are creamy white just after molting (6 mm long). The pupae become tannish-brown just before molting to darkling beetle adults.

BIOLOGY

Distribution -- Lesser mealworms are found in grain bins, mills, and poultry houses throughout the world.

Food -- Lesser mealworms feed in grains and flour, particularly in damp, musty sites. Poultry houses with deep litter are ideal breeding grounds. Adults have been found feeding on carcasses in poultry houses.

Damage -- Lesser mealworms destroy insulation in poultry houses by extensive tunneling. In severe infestations, as much as 25% of insulation may be destroyed in one year. The virus causing Marek's disease of poultry and poultry tapeworms may be transmitted by darkling beetles. Occasionally young birds or sick birds may be attacked by darkling beetles.

Life History -- Lesser mealworms develop more rapidly in warm weather than in cold weather. Eggs hatch in 4-7 days. Tiny lesser mealworms hatch and develop in the litter. Lesser mealworms develop through several stages before they molt into the pupal stage. Larval development takes up to 7 weeks. Mature larvae seek a sheltered place to pupate because the darkling beetles prey on the lesser mealworms. Most of the damage to insulation is done by lesser mealworms seeking a safe place to pupate. The pupal stage lasts 7-11 days. Newly molted adult beetles are tannish-brown and darken slowly to black. A beetle may live two years and females lay up to at least 110 eggs a month.

Darkling beetles are commonly found in woods or around feed bins. These beetles fly well and are attracted to lights at night but hide during the day. The beetles are attracted to poultry operations because of the ideal conditions for their development (warm, humid litter and abundant feed). Certain aspects of the behavior of lesser mealworms and darkling beetles in poultry houses are presently unclear, particularly as to why adults enter the insulation and what can be done to inhibit this movement.

CONTROL

Monitoring lesser mealworm infestations in poultry houses is done with a beetle trap. One type of trap is plastic (PVC schedule 40) pipe 10 inches long with a roll of corrugated cardboard inside. When placed on the litter, lesser mealworms and darkling beetles crawl into the trap. The cardboard can be removed and the insects counted every week or two. Thus by using 3 to 5 traps per house, a producer can at least tell if his beetle population is increasing or decreasing. A rapid rise in the number of lesser mealworms per trap indicates a chemical treatment is necessary to prevent excessive damage to the insulation. By treating before the larvae become adults, the life cycle of the beetles is disrupted and longer control achieved.

Probably the single best strategy for darkling beetle control is frequent litter cleanout. This is not always feasible since total cleanout is costly and time consuming. Also, depending on the time of year, cropland on which to spread the litter is not always available. Before removing infested litter to spread on a field, the litter should be treated with an insecticide 2 to 3 days before it is removed and then disked in after spreading. This will prevent beetles from migrating to nearby homes and other buildings and becoming a nuisance.

In poultry houses with a prior history of darkling beetle problems, chemical control is recommended. Ideally, the litter, posts, and sidewalls to a height of 2 to 3 feet, should be treated as soon as possible after birds are removed from a heavily infested house. In cool weather, after birds are removed from a house, the beetles move quickly away from the surface of the litter. In cool weather, if the temperature in the house is allowed to drop before chemical application, poor control can be expected. It may also be helpful to treat new litter 1 or 2 days before birds are placed into the house. In houses that are half-brooded, chemic al costs may be reduced by treating only the brooded half, since beetle numbers are usually very low in the nonbrooded section.

In houses with birds 8 weeks old or older, chemicals recommended for control may be applied with the birds present. Chemical treatments applied to the litter, however, cannot be expected to provide effective residual control over a long period of time, since most insecticides readily combine with the high organic content of the litter. Insecticides and disinfectants should not be mixed and sprayed together since most combinations are not compatible.

As a cultural control measure, when cleaning out houses during cold weather, the curtains should be completely lowered for several days. This will allow the house to drop below 45 degrees F which kills the beetle eggs. It is also helpful to prevent extremely wet spots under waterers by either using spare waterers while the litter dries or by occasionally changing the waterers' positions.