The sawtoothed and merchant grain beetles are small, long, flattened beetles about 1/10 inch (2.5 mm) in length. These two dark brown beetles are essentially identical to each other and are characterized by the six-saw-like teeth on each side of the prothorax. The difference between the two species is seen by examining the eyes and the area just behind the eyes.
The sawtoothed grain beetle has smaller eyes than the merchant grain beetle. The area just behind the eyes of the sawtoothed grain beetle is much larger than the same area on the merchant grain beetle which appear more like “points”.
Sawtoothed and merchant grain beetles are two of the more commonly encountered insects in processed grain, oats, pet foods and seeds. Other foods attacked include rice, cereals, dried fruits, breakfast foods, grain meals, sugar, chocolate, drugs, pastas and tobacco. Its varied food preferences make it one of the most commonly encountered stored product beetles in retail food stores, warehouses and homes.
Biology and life cycle
Both beetles have nearly identical biologies. One female may produce up to 285 eggs which are laid singly or in small batches in crevices found in the food medium. The larvae that emerge are yellowish white with a brown head and grow to a length of about 1/8 inch (3mm). Larvae are active crawlers and move about through the food as they feed. They are probably not capable of feeding on whole grains unless other insects have attacked the kernels. After molting two to four times, the larva pupates and attaches bits of grain dust and flour to the cocoon. The adults emerge in about seven days, and the entire life cycle can take from 27 to more than 50 days depending on environmental conditions.
Large populations of these beetles can quickly develop and adult beetles will begin to emerge from infested foods to seek fresh breeding sources. In one situation encountered by one of the authors, a home was experiencing hundreds of sawtoothed grain beetles inside the kitchen. Inspection revealed no foods were infested in the kitchen, but a large bag of bird seed in the garage next to the kitchen was heavily infested. The floor under and around this bag was literally black with thousands of dead beetles and the beetles had even crawled by the hundreds inside a refrigerator located in the garage.
In situation where severe infestations are present, sawtoothed grain beetles have been reported to land on and nibble on the skin of people. Although these bites are not dangerous, such activity is certainly disconcerting and unwanted.