Pests are a nuisance and constantly on the search for three things: food, water and shelter. If they find these things, they can make themselves at home and start causing damage. Unfortunately, grain processing, handling and storage facilities offer a prime environment for a pest introduction. There are a variety of pests that can wreak havoc at these facilities and stored product beetles are a prominent threat. These beetles are actively on the hunt for the product and food supply that grain facilities work so hard to protect and can cause immense damage in a short period of time if left unchecked.

According to the U.S. Department of Agriculture (USDA) and a study done at the University of Wisconsin, “stored product pests can damage, contaminate or consume as much as 10% of the total food produced in the U.S. alone, while in developing countries that rate has been estimated at 50% or more.”

Beetles belong to the largest order of insects with more than a 250,000 beetle species currently identified in the world. In North America alone, scientists have identified more than 25,000 species. However, there are three types of beetles that food processing facilities need to be aware of as they pose some of the biggest threats: lesser grain borers, weevils and flour beetles. All three are small, reddish brown and feed, reproduce and survive right there in the product. The best way to defend against and help diminish these pests and the threat they pose is to learn about them. This way, you can help prevent an infestation from occurring or getting significantly worse.

Let’s dive into the biology of three of these beetles and the specific concerns they can cause. 

Lesser grain borers

In the United States, lesser grain borers are considered the No. 1 damaging pest in stored wheat. These are small, reddish-brown beetles with a rectangular shape and a head that is tucked down. They have deeply pitted outside wing covers (elytra). These insects are internally developing insects, which means they need a full, intact kernel of grain (usually wheat) to go from the egg to the adult stage. Females lay their eggs outside the kernels on the fines and broken grains and as soon as the larvae hatch, they tunnel their way into an unbroken kernel, where they feed. As the larvae develops, it eats the entire inside of the kernel, leaving nothing but the outer shell. It even pupates inside the grain. Once it is an adult, it chews a round exit hole and exits to start the process all over again. Lesser grain borers are difficult to detect because only the adult stage is visible so there’s no telling how much of the grain is actually infested, but samples can be taken to look for insect damaged kernels (IDK). High infestations of this insect can lead to “hot spots” in grain masses where the temperature rises above the rest of the grain mass. The increased temperature and broken kernels can lead to other insects taking advantage of the new food source.


Three common species of weevils in the United States are the rice weevil, maize weevil and granary weevil. Don’t let the names fool you: rice weevils will feed on more than just rice, and maize weevils will feed on other grains besides corn. These are small, reddish-brown insects, somewhat rectangular in shape with their head prolonged into a “snout” distinctive to weevils.

Like lesser grain borers, these are internally developing insects. Unlike lesser grain borers, they lay their eggs inside the kernels. So, their entire development from egg to adults is protected within a kernel of grain. They do leave the same distinctive exit hole that can be used to identify infestations. As with all insects, the time to develop from egg to adult is based on temperature and food resources but they can develop as fast as five weeks.

Flour beetles

Flour beetles are small, so it usually takes magnification to tell the difference between the red flour beetle and the confused flour beetle. However, both species are reddish brown and more rectangular shaped.

Flour beetles are a common pest found in grain and grain-based products. The female beetle deposits eggs on broken kernels of grain and fines. The larvae hatch and can develop from egg to adult in as little as one month. They can live for nearly a year as adults and deposit hundreds of eggs in that time span.

The most obvious sign of these beetles is the sighting of small rectangular beetles near or in the product. The beetles take advantage of internally developing insects such as weevils and lesser grain borers who break up the grains for the flour beetles to then feed on. These, like other grain infesting insects, can raise the temperature and create moisture issues leading to mold problems. And that’s on top of the actual physical damage they do to the grains that can cause dockage and reduce value. 

Prevention tactics

Now you know what to look for with these beetles, let’s talk prevention. There are a variety of tactics you can implement within your pest management plan that may also help meet government and audit requirements.

The first step to implement is to sample all incoming grain shipments for insects and IDK. This way, you are not bringing in infested grain and contaminating grain that you already have in storage. Excess fines, dockage and moisture also can be a sign of a potential problem. Also, while sometimes challenging, store all new, incoming grain separately from older grain.

Another tactic that can be implemented to prevent stored product beetles is monitoring temperature. Hot spots identified in the grain can mean an insect issue, in which those grain bins can then be treated. Temperature probes also can be combined with monitoring probes to detect adult insect activity. Also, these beetles are temperature dependent. Most stored product insects can’t develop below 13 degrees C (55 degrees F). If aeration is possible to cool the grain on cool nights or in cooler seasons, insect feeding and developing can be halted.

Routine sanitation is another great tactic to prevent pests and minimize their damage. Everyone should be clear on roles, responsibilities and expectations. Cleaning up product spills immediately and looking out for wet or damp spots are key to preventing mold and creating an environment where pests can thrive. Making sure grain is free of dockage and fines as it is being placed into storage will minimize insect pests as well.  

Beetles are capable of destroying your product and costing you money, safety and your reputation, so focus on prevention and early detection as part of your current integrated pest management plan.