Of the more than 30 species of beetles that infest stored products, only a few are considered serious pests. Some, like the rice weevil and lesser grain borer, are far larger pests in farm-stored grain than might be seen in food-processing facilities. Cigarette, Trogoderma and flour beetles are far more common in food processing, while drugstore, cigarette, Trogoderma and sawtoothed grain beetles are seen most often in warehouses and retail stores.
As PMP Hall of Famer and my former professor, Dr. John Osmun, used to say, “Start with the insect.” If you know the identity of the target pest, you know its biology and, thus, its preferred conditions for harborage or breeding. In the case of stored product beetles, examine monitoring traps, insect light traps (ILTs), or stored products themselves to find the beetle(s) to identify. On occasion, the customer will provide a specimen to identify.
Stored product beetles may be found breeding in one of several locations: in raw stored grains or grain-based products in bins, in packaged materials, in spilled grain, seeds, or grain-based product, and in grain/flour dust accumulated on ledges, inside electric boxes, or in equipment. Rarely, beetles that fly like Trogoderma beetles invade from outside sources.
Another aspect to consider is that some beetles are fliers, so they may be found far from where the actual infested source is located. Sawtoothed and merchant grain beetles closely resemble one another, and only the latter species flies. Red flour beetles can fly, but their close cousin, the confused flour beetle, does not. Non-flying species will typically be found closer to the breeding source, thus saving time in the inspection process.
The type of food infested by each beetle can be helpful for those species with more specific diets. For example, bean weevils only infest whole legumes, so where are the whole beans stored? Internal feeders such as rice weevils and lesser grain borers prefer whole grains and seeds, so check these items. Of course, with insects come exceptions to the rule: Rice weevils are partial to older stocks of stored pasta, and lesser grain borers are known to infest old flour and similar processed grain.
Many external feeders are known to infest the widest variety of products, and can be the hardest food pests to pinpoint breeding sources. These include cigarette and drugstore beetles, as well as the several species of Trogoderma. These latter beetles also feed on dead insects in ILTs and on windowsills, and can feed on the hair and feathers of rodent or bird carcasses in wall voids and false ceilings.
Scavengers such as sawtoothed and merchant grain beetles and flour beetles infest processed grain products or whole grains being damaged by internal and external stored product pests. Sawtoothed grain beetles are very common in birdseed infested by other insects. Secondary pests need moist or moldy grain or grain-based products to survive. In food processing facilities, be sure to check under conveyors (especially those leading from grain bins), and look for spilled product collected into expansion joints, the bases of storage racks and similar sites. Another source is the bottom edges of grain storage bins and older stock of products in warehouses that has not been rotated.
Inspections to find sources of stored product beetles can take considerable time, particularly in food processing facilities and warehouses where grain-based products may be widely distributed throughout the facility. By focusing on the type of beetle present and its preferences and biology, time can be saved. An important point is to not stop when one source is found: Expand your search from there to determine whether other items are infested or other sources exist (such as spilled grain).