Figure 1. Grain of wheat (top left); adults of sawtoothed grain beetle (top right), red flour beetle (bottom left), and rusty grain beetle (bottom right). Note how these are much smaller than the grain of wheat.
Insects found in farm-stored grain can include those that feed directly on the grain, and those that feed on fungi growing on the grain. Knowing the identity of insects found in stored grain is critical for determining whether control is needed and what methods would be most economical.
Insects that feed directly on the grain include rusty grain beetle, red flour beetle, sawtoothed grain beetle and larvae of the Indian meal moth.
Rusty Grain Beetle (Cryptolestes ferrugineus) is the most common insect that feeds on stored grain in Manitoba. The adult beetles are rectangular, reddish-brown, less than 2 millimetres long, and have long antennae that look bead-shaped when the insect is magnified (Figure 1). Larvae are worm-like, white, and have 2 brown projections on the tail end. Heavy infestations of rusty grain beetles can cause grain to heat and spoil.
The red flour beetles (Tribolium castaneum) is the second most common insect pest of stored grain in Manitoba. Adults are also reddish-brown but are about 4 millimetres long, about twice the length of the rusty grain beetle. Larvae are white with pale brown bands. The red flour beetle cannot feed on undamaged, dry seed with less than 12% moisture content. It prefers grain dust, broken grain and milled stocks. Red flour beetles can be controlled by moving grain, whether or not it is cold.
Adult sawtoothed grain beetles (Oryzaephilus surinamensis) are brown, about 3 millimetres long, and when magnified you can see six tooth-like projections on each side of the thorax. Sawtoothed grain beetles are more common in stored oats than in stored wheat and barley. When found on a mass of whole grain, its presence indicates that either the grain was invaded earlier by other grain feeding insects or the grain mass contains broken grain, weed seeds, dust, or other types of dockage, because this insect species does not feed on undamaged grain.
Larvae of the Indian meal moth (Plodia interpunctella) construct tunnels of silk and droppings through stored grain, as well as feeding on the grain. They have been found on stored corn in Manitoba.
Insects and mites that feed on fungus in stored grain include the foreign grain beetle, psocids and grain mites.
Adult foreign grain beetles (Ahasverus advena) are brown and about 2 millimetres long. Although in many ways they resemble rusty grain beetles, the antennae of foreign grain beetles are club-shaped (Figure 2), although you may need to magnify them to see this. Also, the foreign grain beetle has a blunt point on the front corners of the thorax (the region behind the head). When placed in a glass jar, foreign grain beetles will climb up the sides, while rusty grain beetles cannot. Although foreign grain beetles are considered a fungus feeder, they will feed on grain if the moisture content is in the high end of the acceptable range (eg. 14.5% mc wheat). Larvae are wormlike, cream-coloured and reach a length of 3 millimetres before pupating.
Figure 2. Foreign grain beetle.
Grain mites are whitish, soft-bodied and about 0.2
to 0.5 millimetres long. They are difficult to see with the naked
eye. Several kinds of mites can occur in farm granaries and
elevators. Some of these are fungus feeders, while others are
predators of other mites as well as insect eggs and larvae. Under
unfavorable conditions, some grain mites go into a nonfeeding stage
which is resistant to drought, heat and even fumigation. They can
remain in this stage for months until favourable conditions activate
them. Moisture content of the grain influences mite activity. If
relative humidity stays between 80-90% they can develop, even under
relatively cool conditions. The best way to control mites is to dry
the grain or turn it from one bin into another.
Psocids are soft-bodied insects, about 1 millimetre long, with long antennae relative to the body size.
Monitoring Pest Levels
Stored grain insects can be monitored either with traps, or by taking grain samples with a bin probe and then visually inspecting the samples. Traps for stored grain insects are long cigar-shaped tubes that are stuck into the top of the grain sample. For some traps there are pheromone attractants available, which may improve trapping efficiency. Regular inspection with traps or probe samples will indicate potential problems that may be developing in the bin.
Be sure the grain is in good storage condition by:
Chemical control of fungus-feeding pests is not necessary, since they cannot survive in sound, dry grain.
Grain-feeding insects are treated differently from fungus-feeding insects. There are 4 main options for controlling insects feeding on stored grain.
1. Cold Temperatures
An effective method of controlling insect infestations in stored grain in winter is to lower the temperature of the grain. Rusty grain beetles and most other stored grain insects can be killed by periods of low temperatures as follows:
|Grain Temperature in degrees Celsius||Time Required to kill insects|
|-5 C||12 weeks|
|-10 C||8 weeks|
|-15 C||4 weeks|
|-20 C||1 week|
Grain in bins over 6 metres in diameter will not cool sufficiently without assistance, such as through aeration, to control some insects.
2. High Temperatures
All four stages of insects (eggs, larvae, pupae and adults) will die if subjected to high temperatures for a sufficient period of time. The most realistic use of high temperatures for controlling insects would be when the grain was tough and in need of drying. In this case, the insects would be killed at the same time as the grain is dried. The insects need to be exposed to temperatures in the range of 50 to 55˚C for approximately 15 minutes.
3. Moving Grain
The use of pneumatic conveyors to move grain has been shown to give good control of some insect pests of stored grain.
Chemical treatment of the bin is another option.
Diatomaceous earth can be used for control of grain feeding insects in many stored grains. It is sold under the trade names Protect-It or Insecto.
Fumigants containing aluminum phosphide, such as Phostoxin, are available in pellet or tablet form. These fumigants should be used only when the grain temperature is above 4oC. The use and sale of aluminum phosphide in Manitoba is restricted to licensed pesticide applicators possessing a valid stored agricultural products license.
Refer to the Guide to Crop Protection for more details on insecticides that can be used to control insects in stored grain.
For further information, contact your GO Representative.