The yellow mealworm beetle (Tenebrio molitor) is an insect belonging to the order Coleoptera, family Tenebrionidae. It is a widespread parasite worldwide, known for attacking food supplies, especially flour and its derivatives. For this reason, this insect is commonly referred to as flour worm. In addition to industrial warehouses dedicated to food storage, it is easily found in the pantries of ordinary households.
In this article, we will see how to recognize this particular mealworm, its feeding habits, the damage it causes to food, its life cycle, and most importantly, how to remedy its infestations.
Description of the Yellow Mealworm Beetle
The adult Tenebrio molitor has the typical appearance of a beetle, measuring 15-20 mm in length. It is shiny reddish-brown, with a flattened head and a hexagonal outline. It has segmented antennae, with the last segment significantly longer. The pronotum (the thorax) is sub-square, much wider than long, with the anterior edge curved inward and the posterior corners protruding. It is also punctuated, with a median longitudinal depression. The elytra (the wings) are quite elongated, with parallel stripes and punctuations. This beetle is a skillful flyer, explaining its cosmopolitan distribution in search of new food sources.
The major damage caused by the yellow mealworm beetle is due to the larval stages, which, in appearance, resemble true worms. These larvae measure between 20 and 35 mm in length at full maturity, with an elongated cylindrical body, a leathery texture, and variable color from initial white to the yellowish and brown of full maturity. At the thorax, the mealworm beetle is equipped with very short legs, which allow it excellent mobility in pantries.
The eggs of the yellow mealworm beetle are tiny (about 1.5 mm in length), sub-reniform in shape, milky white in color, with a smooth and sticky chorion.
Damages Caused by Yellow Mealworm Beetles
Adults and especially the larvae of the mealworm beetle live by feeding on wheat flour and its by-products, including bread, pasta, biscuits, etc. However, infestations have been observed to affect intact grains (caryopses) and other foods such as rice, bran, barley, oat flakes, spices, leather, and more. Food attacked by yellow mealworm beetles acquires a foul odor and is contaminated with insect excrement, molted exuviae, and the corpses of dead insects. As indirect damage, there is a possibility of allergies due to inhalation of insect exoskeleton fragments, which, of course, affects people working in large grain and flour storage warehouses.
Cultivation of Tenebrio molitor Larvae
It is worth noting that yellow mealworm beetles are increasingly being cultivated, as they are excellent food for captive animals (birds, reptiles, fish etc.) and widely used as fishing bait. In Asian countries, they are consumed by humans, and in Europe, with a May 2021 regulation, their use as human food has been authorized.
Adult yellow mealworm beetles usually appear in spring. Being excellent flyers, they easily reach food sources, including warehouses, mills, homes, and pantries—basically, anywhere there are starchy and food supplies. The adult lives for about two months and is highly resilient, capable of surviving even without nutrition. Its preferred refuges are floorings and wall crevices. After mating, females lay eggs, ranging from 150 to 500, depending on the richness of food sources.
Development of Flour Beetle Larvae
The larvae of Tenebrio molitor hatch after about 5-15 days of incubation and undergo very slow development. If they are numerous and food is scarce, they start feeding on each other. Full maturity is reached after 160-180 days and 8-10 molts (at temperatures above 25 °C). If temperatures are below 20 °C, the maturation time of the larvae significantly lengthens and can take up to a year. Once development is complete, flour beetle larvae climb the surface of the attached substrates and pupate. The new adults are ready to emerge after 8-13 days. Normally, only one generation is produced in a year.
How to Prevent Yellow Mealworm Beetle Infestations
To prevent serious infestations of yellow mealworm beetles in domestic settings, periodic inspections in pantries and warehouses are necessary. Furthermore, it is essential to ensure the absolute cleanliness of these places. It is a good practice to store flour and derivatives in shelves and cabinets with airtight closures and to clean them regularly. Never leave packages open; it is best to consume them quickly or, at the very least, seal them. In a sterile environment, flour beetles have no means to reproduce.
Where these precautions are not feasible, an effective insecticide repellent can be used, one that is absolutely non-toxic, such as diatomaceous earth (or fossil flour). By spreading the powder in corners and along the baseboards of the room, infestations of not only yellow mealworm beetles but also ants, grain weevils, cockroaches, bread beetles, and bugs can be prevented. Diatomaceous earth is an approved product in organic agriculture (and can be found here).