Red flour beetle

Tribolium castaneum Herbst

Coleoptera: Tenebrionidae

Synonyms for Tribolium castaneum are Colydium castaneum Herbst and Tribolium navale Auct.

Common English Names: Red flour beetle, Red grain beetle, Red meal beetle.

Host plants and damage

T. castaneum has a broad host range and attacks mainly dried stored products. Hosts are, Phaseolus (beans), Pisum sativum (pea), Oryza sativa (rice), Zea mays (maize), Triticum (wheats), Avena sativa (oats), Hordeum vulgare (barley), Secale cereale (rye) and others. It is found in the filed feeding on mature seeds. The species is cosmopolitan, but is especially damaging in tropical, subtropical and mediterranean climates.

Plant growth stages affected

Affected plant stages are roots, seeds, and fruits/pods.

Affected plant parts

Stages infest fruits, pods and vegetative organs during post-harvest.

Damage

Larvae develop on undamaged grain and prefer to feed on embryos. If insect numbers are low, detection is difficult, but high infestations can be readily observed by boreholes and when beetles move through the stored product. Damage of T. castaneum is serious because seeds are partially consumed and the endosperm becomes flour-like in appearance.

Morphology & biology

The reddish-brown beetles are 2.3–4.4 mm long, flat and have an oblong shape. The head and upper thorax are covered with punctures. Furthermore, wings have ridged, lengthwise punctures.

Females of T. castaneum lay up to 450 eggs in stored products. The eggs are approximately 0.5 mm long, white and cylindrical. They are covered with a sticky secretion and flour, which makes them difficult to detect. The incubation period of eggs takes 5 to 12 days.

Larvae are yellowish-white, cylindrical and covered with fine hairs. The head is pale-brown and the last segment of the abdomen has two upturned dark, pointed appendages. The larval stage lasts 27–29 days, and the last instars can reach a length of 6 mm.

Pupae have no cocoon. They are yellowish-white and later become brownish. The dorsum of pupae is hairy and the abdomen ends in two spines.

Adults emerge from the pupae in 3-7 days. The life cycle can be completed in 20 days under optimum conditions of 35 °C and 70 % relative humidity, but requires 141 days at 25 °C and 70 % relative humidity. Beetles may live as long as 18 months, depending on environmental conditions. The upper and lower threshold temperatures for development of T. castaneum in rice at 70 % relative humidity are 40 °C and 20 °C, respectively.

Adult beetles fly in the field at temperatures at 26 °C. They produce sex and aggregation pheromones.

Adults and larvae are very similar to those of T. confusum, although the adults of T. castaneum are slightly smaller. Distinguishing features between the species is written under T. confusum.

Monitoring

At low levels of infestation, holes or tunnels in seeds may not be detected. However, when infested seeds are cracked open larvae can be found inside. A non-destructive method to inspect seeds is to use an X-ray machine to view bore-holes on film. In contrast, at high levels of infestation, bore-holes can be seen and adults are present. The host, especially flour, becomes grayish-yellow and mouldy, with a musty odour. Trapping techniques can be used to continuously monitoring populations, even at low infestations. Adults respond to aggregation pheromone as well as to sex pheromone traps. Unbaited traps, such as probe traps or pit fall traps can also be used to detect infestations of T. castaneum.

Pest management

Gamma irradiation of infested stored products and exposure to temperatures below 10 °C reduce pest numbers. Low oxygen and carbon dioxide-enriched atmospheres can be used to control T. castaneum.

Chemical Control

Pyrethroids, organophosphates and fumigants have been used for chemical control of T. castaneum. The species has developed in some countries resistance against malathion, chlorpyrifos methyl, dichlorvos and phosphine

Under hot dry conditions surface treatment with diatomaceous earth can prevent infestation.