Stored Product Pests: Did They Ride In, Or Are They Breeding Here?

Beetles and moths in packaged goods come from one of two places. They rode in inside a lot of product, or they are breeding in spilled material that the cleaning schedule never reaches. Telling those two apart is the whole job, because one is a receiving problem and the other is a housekeeping problem, and treating the wrong one changes nothing.

Northeast Ohio is full of buildings where this matters: food plants, contract packers, third party distribution centers and dry storage warehouses across Stark and Summit counties. The insects involved are small, they are easy to mistake for each other, and most of them spend the part of their life that matters somewhere nobody is looking. Our warehouse and manufacturing pest programs are built around finding the source rather than counting the insects on the floor.

Which Insects Actually Get Into Packaged Goods?

A handful of species account for nearly everything found in dry storage. They sort cleanly by what they can chew, and that tells you where to look for them.

Size is part of why these go unnoticed. Purdue University Extension puts the sawtoothed grain beetle at about 2.5 mm, roughly a tenth of an inch, with a very flat body that it describes as well adapted for crawling into cracks and crevices, and six saw-like projections along each side of the thorax. It is separated from the closely related merchant grain beetle by a small projection on the head behind the eyes. Virginia Cooperative Extension calls the same beetle a common pest of homes, grocery stores, food warehouses and grain storage facilities, which is a fair description of how far it travels.

Did It Ride In, Or Is It Living Here?

Entomologists split these insects into primary and secondary pests, and that split answers the question in the heading. Colorado State University Extension defines primary pests as the ones that feed within intact kernels, and secondary pests as the ones that feed on broken kernels or grain dust. The LSU AgCenter describes the primary group's larvae as feeding and developing inside the grain and then chewing their way out, and the secondary group as feeding externally on broken or damaged grain and powdered products.

That has a practical consequence. If the problem is a weevil or a lesser grain borer, the immature stages were sealed inside kernels when the lot arrived, so a receiving inspection that looks at the outside of a bag can pass it without a second thought. If the problem is a flour beetle or a sawtoothed grain beetle, the material it needs is broken, milled or spilled, and a building generates that on its own every day.

It is tempting to conclude that the secondary species cannot start anything by themselves, and the High Plains IPM Guide says flatly that this is a misconception: in almost any storage situation there is enough broken grain and debris to support an infestation by secondary invaders. Penn State Extension puts the sawtoothed grain beetle in elevators, mills, processing plants, warehouses and kitchens, feeding on broken kernels and grain residues. In other words, a clean inbound lot does not protect a dirty floor.

Why Traps Beat Opening Boxes

The clearest published statement on this comes from the Armed Forces Pest Management Board, whose Technical Guide 27 covers stored product pest monitoring. It calls monitoring with pheromones and food attractants the most useful technique currently available and the key component of an integrated management program, and it is blunt about the alternative: product and visual inspections are labor intensive, and product inspection depends heavily on chance when an infestation is at a low level. Technical Guide 5 from the same board explains why the traps get used in the first place, which is that it is hard to tell which product is infested, and notes that placing traps in a grid lets a facility be covered systematically.

The lure has to match the insect. Technical Guide 27 specifies traps baited with aggregation pheromone for flour beetles, and notes that beetle traps are generally baited with a food attractant as well to improve capture. A grid of traps read over weeks does something a single inspection cannot: it shows you a corner of the building where the count keeps climbing, which is a far better description of where the problem lives than a list of species.

The Two Places Almost Nobody Checks

Two spots show up repeatedly in the published guidance and are easy to leave out of a routine.

• The middle of a pallet. Technical Guide 38 recommends removing a layer or two of packages from the top of a pallet and checking the surfaces and the folds between packages. The outside faces of a stretch-wrapped pallet are the only part most people ever see, and they are the part least likely to show anything.
• Rodent bait stations. The same guide advises checking them periodically for insects, because rodenticides combined with a grain base often become infested themselves. A station installed to solve one problem can quietly become a breeding site for another, which is one reason a rodent program has to be serviced and documented rather than simply installed.

Utah State University Extension adds a third, which is spilled food sitting in cracks, crevices and floor seams, including inside shipping containers, rail cars and trucks. The trailer that delivered a torn bag last month is part of the facility for this purpose.

The Warehouse Beetle Is The One To Get Identified

One species deserves separate treatment. Utah State University Extension describes warehouse beetle larvae as roughly a quarter inch long, orange brown, with sharply protruding hairs, and notes they turn up in spilled food in floor cracks and seams. The Defense Centers for Public Health fact sheet on the species explains why it is hard to finish: when food becomes scarce the larvae can enter a long period of inactivity called diapause, and that, combined with an ability to chew through packaging, makes control difficult. The same sheet notes that the shed skins and hairs can irritate the respiratory and digestive tracts of anyone exposed to them, which matters in a building where people work beside the product.

There is also a regulatory reason not to guess. Utah State University Extension says that because the warehouse beetle closely resembles the Khapra beetle, a federal quarantine pest, a suspected specimen should go to a pest management professional or an extension agent for identification. USDA APHIS has restricted commercial and noncommercial shipments of rice, soybeans, Cicer species such as chickpeas, and safflower seed arriving from countries where the Khapra beetle is known to occur. In that situation, a beetle logged under the wrong name is more than a paperwork problem.

What Actually Ends It

The published recommendations are consistent and unglamorous. Virginia Cooperative Extension puts the first step as careful inspection and discarding heavily infested material. Utah State University Extension lists the rest:

• Inspect incoming items for pests before they join the rack, and open past the outer layer of the pallet.
• Clean up spilled product quickly, especially in storage areas, and get into the floor seams rather than over them.
• Keep moisture low in storage areas by improving ventilation.
• Store susceptible items in pest proof containers rather than in the bag they shipped in.
• Dispose of infested material instead of moving it to a back corner to deal with later.
• Inspect and clean the vehicles used to move food, so spilled material does not ride back and forth.
• Monitor with a lure and trap, so the next population is found while it is still small.

Notice what is not on that list. Iowa State University Extension states plainly that insecticide sprays are not recommended for insects in stored food cupboards, and the logic scales: the insects are in the material, so a treatment that leaves the material in place has not removed the thing producing the insects. In a facility that reasoning is what separates a program that removes the cause from one that just closes a service ticket. That distinction is the point of our commercial IPM work, and it is the same discipline behind pest control in a food plant. The home version of this problem, the pantry rather than the pallet, is in our post on pantry moths in Ohio kitchens.

If beetles or moths are showing up in your dry storage and nobody can say which lot they came from, that is the question worth answering first. Call 330-494-6199 or ask for a free estimate, and we will walk the building and the dock with you.