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Bug of the Month Nov. 2019


Pests of stored grain

by Vincent Hervet and Paul Fields

Grain stored in bins can be infested with seed feeding insects. To maintain high quality grain, there is zero tolerance for live insects in grain sold into the commercial grain handling system.

There are over a 100 different stored grain insects in Canada, but only three are commonly found in grain bins on farms: the rusty grain beetle, the red flour beetle, and the foreign grain beetle.

The rusty grain beetle and the red flour beetle feed on grain, particularly damaged grain. The foreign grain beetle is a fungus feeder, and its presence in bins is associated with moldy grain. The small size and flat shape of these beetles enable them to crawl through gaps between kernels, making them difficult to detect. The pictures below display beetles on kernels of wheat for size comparison.

Rusty grain beetle adult
Rusty grain beetle adult on a kernel of wheat.
Red flour beetle
Red flour beetle adult on a kernel of wheat.
Foreign grain beetle
Foreign grain beetle adult on a kernel of wheat

According to the Canadian Grain Commission, 90% of the detections in terminal elevators are the rusty grain beetle, 5% are the red flour beetle, and the reminder various other insects.

High fertility and a rapid development rate enables this insect to quickly increase its numbers, up to 60 times per month under ideal conditions. This allows it to rapidly increase in the warm grain after harvest. Adults can live for over a year and are often able to survive the winter in the centre of unaerated bins.

Red flour beetles also develop rapidly, but die during the augering of grain and are less cold tolerant than the rusty grain beetle. Infestations of this insects can be controlled by lowering the grain temperature soon after harvest with aeration. The development rate of all species increases with higher grain temperatures and moisture contents. Their development and reproduction ceases below 18 °C. Fumigation with phosphine is another control method, and requires licensed operators for application. 

convection currents in bin-stored grain
The cycle of convection currents in bin-stored grain when ambient air outside the bin is cold and the grain is warm

In the absence of – or with too little – air drying, the natural cycle of air in grain bins makes moisture migrate to the centre and top of bins (see diagram below). This can create a hot spot where insects and molds thrive. Use aeration to cool and dry the grain. Further information on the prevention of spoilage of grain in bins can be found at: