Insect Identification sample sheet
Mealworm Beetle
The mealworm is the larval stage (grub) of the beetle Tenebrio molitor and is so-called because it is a pest of grain and grain products. It occurs frequently in the home where it may cause alarm due to the size of the larva and the adults' habit of flying after dark, when it may be attracted to light. It is not a significant pest of stored-food products owing to its slow rate of development and is not a threat to health. Larval stages are not known to damage wood.
Identification
Adult beetles are dark brown to almost black in colour and range in length between 12 and 20 mm. When viewed from above, the head, thorax and wing-cases are smooth and shiny, without hair. A less common closely related species occurs in the UK but this has a matt black colour. Larvae of Tenebrio molitor, when fully grown, are reddish-brown, smooth and often up to 40 mm long.
Distribution and life-cycle
This species occurs all over the UK and is also found throughout the temperate regions of the world where it can develop at relatively low temperatures. It does not occur naturally in the tropics. In common with other beetles it passes through four life-stages: egg, larva, pupa and adult. Developmental times vary according to differing environmental factors, such as dietary quality, temperature and moisture availability. Optimum developmental temperatures are between 25° and 27°C but eggs can be laid at the lower limit of 14 C. Under normal conditions in this country one generation is produced per year, but if higher and more favourable temperatures exist then development is rapid and two generations may emerge. It is likely, however, that at temperatures consistently above 30°C breeding is not possible and this may account for its exclusion (except from in imported cargoes) from tropical regions.
Control
The only way to effectively eradicate the beetles is to remove and destroy the food-source of the larvae. Often household infestations originate from birds' nests or associated debris in roof spaces or attics. Occasionally pigeon nests etc. in the roof have been locations from which larvae have dispersed, subsequently falling into water tanks and then appearing in bathroom water, much to the occupier's alarm. Any other pests that might be providing food-material in the form of droppings, nesting debris, corpses etc. should also be eradicated and their locations thoroughly cleaned. Access to nest sites in the roof should be blocked to prevent future use. Birds also often drop bread and other food down chimneys, so this may be the source of beetles that have appeared in fire-places in rooms without any apparently available food supply. Such fire-places should be exposed and thoroughly cleaned out where necessary. Providing that all potential food-sources are sealed, infestations are unlikely to persist under normal conditions of prudent hygiene. Any stray wandering adults will be deprived of the food needed to develop the next generation.
Contact points
This information has been produced by the Insect Information Service of the Natural History Museum. For details of our advisory and identification services, please contact:
Enquiries Manager
Insect Information Services
Department of Entomology
Natural History Museum
Cromwell Road
London, SW7 5BD
Telephone: +44 (0) 20 7942 5726
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