Description and Biology

The mealworm larva is yellowish to brown with a smooth, shiny, wormlike body. It is about 1/8" thick and is usually 1 to 1/4" long when mature and ready to pupate. The emerging adult (beetle) is black and about 3/4" long. After mating, the female can lay up to 1,000 eggs which hatch in less than 18 days. The newly hatched larva takes six to nine months to mature. The life cycle (egg to egg) usually spans somewhat more than a year; however, under ideal conditions it can be completed in as little as four months.

Mealworms can be found in moist grain in dark and undisturbed areas. The normal operations of feed grain systems would not suit them, but waste, dusty, or damp areas create a favorable haven. Although they feed on cereal products used in homes, such an environment ordinarily would not meet the insect’s needs.

Mealworm outbreaks usually occur where grains are processed and stored. Litter remaining in old or abandoned poultry houses appeals to these insects, and explosive populations have created severe problems for residents within 1/2 mile of the source of an infestation.

Management

Early detection through good management is important. Keep the food in a tightly covered container and clean containers frequently. Do not overlook dry dog or cat foods.

Infested grain must be removed from storage and disposed of or used. Before it can be used, the grain must be treated with insecticide. Waste products must also be treated unless they are buried in a sanitary landfill. Infested poultry litter must be handled the same way unless it is spread over the ground and plowed under. Poultry litter simply dumped outside on the ground can become infested by beetles scattered in the area. Call the Pest Management Office for recommendations on treating large quantities.

Those bothered by mealworms should remember never to discard infested food without first killing the insects. This is best done by placing the infested food in a container and heating it in an oven at 130° F to 150 °F for 30 minutes long enough to reach this temperature. Then discard the food. Placing infested food in a freezer for five days or so should also kill the insect in all stages of development. A vacuum cleaner can be an effective tool. Discard bags sealed in another plastic bag to help prevent spread of infestation. A household aerosol insecticide can be used inside the home, but the results may be disappointing because other mealworm beetles may come in from outside.

Adult mealworms outdoors must be controlled at the source. If mealworm adults are a continuing problem, a 5 foot band of spray can be applied around the foundation of the house. Malathion, carbaryl (Sevin), or a spray mixture containing these two insecticides will help deter invading beetles. Sprinkle enough water on the treated area to wash the insecticide off the grass and onto the ground where the beetles commonly hide in the daytime. These nocturnal insects will also fly to lights and hit the house, which knocks them off balance and causes most of them to fall into the sprayed area. This should give them enough of an exposure to the insecticide to kill them before they can find their way into the house.