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Mullen's Musings™


Insect of the Month, September 2004

Red Flour Beetle, Tribolium castaneum (Herbst)
Confused Flour Beetle, T. confusum Jacquelin du Val

The red flour beetle and its first cousin, the confused flour beetle, are common cosmopolitan pests of stored foods. The red flour beetle is generally found in warmer climates, while the confused flour beetle is found in cooler climates. They are difficult to distinguish and were once thought of as the same species (hence the name "confused"). The antennae of the red flour beetle have three enlarged segments, and the eyes of the confused flour beetle are tucked down under its head. The red flour beetle is capable of flight, but the confused flour beetle is not.


Both species are similar in appearance and habits. They are small reddish beetles about 4-5 mm long. The antennae of the red flour beetle are clubbed, with the last three segments about the same size. The last four segments of the confused flour beetle antennae are enlarged, but tapered. Both are very active as adults and may live two to three years, breeding throughout the year.

The eggs, which may be laid at the rate of two to three per day, are small and whitish. They are covered by a sticky material and are usually found with flour particles adhering to them. Over a lifetime, a female will usually lay between 300 and 400 eggs, and sometimes more. Depending on temperature, the eggs will hatch in five to 12 days. Under favorable conditions, about 90% of the eggs will hatch.

Mature larvae are about 6 mm in length, white to yellow in color and the last body segment has a two-forked termination. The larval instars may vary from five to eleven, but there are usually only seven or eight. The number of instars is influenced by food, temperature and humidity. The larvae are negatively phototactic, but are generally found feeding near the surface or in cracks and crevices. Fully grown larvae will come to the surface to pupate.

Pupae are naked and lack any protective covering. The pupal stage may last only four days under optimal conditions (35 to 37.5 degrees C, 70% relative humidity), but will last for a significantly longer period as the temperature cools.

Flour beetles feed on a variety of foods, including grain, flour and cereal products, as well as peas, beans, cacao, cottonseed, nuts, dried fruits, drugs, spices and dried milk. They are not capable of feeding on sound grains, but only on broken kernels. The red flour beetle is a major pest of stored peanuts.

The flour beetles are secondary pests and are often found in association with Sitophilus sp. (Maize, rice and granary weevils) and the lesser grain borer. In large numbers the flour beetles produce quinones that cause a characteristic odor that is often associated with infestations and which may give the infested product an off flavor.

The entire flour beetle life cycle can last from 19 to 150 days, and is dependant on diet, temperature and humidity. At room temperature, female red flour beetles and confused flour beetles live for an average of 226 days and 447 days, respectively. Under the same conditions, the red flour beetle female lays 327 eggs and the confused flour beetle lays 458 eggs.

The pheromone is an aggregation pheromone (4, 8-dimethyldecanal). Unlike sex pheromones, both sexes are attracted to it. The pheromone serves as a signal that both food and mating opportunities are available. When populations are high, quinones are produced that counteract the effect of the pheromone. The most effective trapping strategy for these insects employs the pheromone with an oil-based kairomone, such as the one used in the Dome Trap. The kairomone is attractive to the adults from a very short distance and also acts as a killing agent.


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