Stored Product Insects – Chewing Their Way Into Our Lives
Written By: Patrick Kelley, BCE, President of Insects Limited
If you listen carefully, you can almost hear the crunching, munching, rasping and chewing of those tiny, stored product insects working their ways through packaging materials into our food and personal belongings.
Even though they are diminutive in size (most stored products insects range from 1 – 6 mm in length), this particular set of insects has become quite specialized in using their mouthparts or body design to navigate through plastic and paper packaging.
Once they penetrate an outer wrap, they find their food source and proceed to consume it to the point where it can no longer be eaten or used by people.
A closeup of the chewing mandibles of a red flour beetle, Image taken from the National Institute of Plant Health Management, Department of Agriculture and Farmer’s Welfare, Ministry of Agriculture & Farmer’s Welfare, Government of India
Not all stored product insect mouthparts are created equal. In this article we will look at three groupings of stored product insects; top-tier penetrators, mid-tier penetrators and low-tier penetrators. Leading the list of the strongest chewers are a group of insects that feed on the inside of hard grain kernels.
This group includes; rice weevil, Sitophilus oryzae, maize weevil, Sitophilus zeamais, granary weevil, Sitophilus granarius, lesser grain borer, Rhyzopertha dominica, and larger grain borer, Prostephanus truncatus.
Strong and resilient mandibles in adults and larvae of these top-tier penetrators are needed to cut and break through the durable husks and hard interior of grain kernels. Insects that can bore their way into a kernel of grain have no problem chewing through thin plastic or medium thickness cardboard to reach their food source.
A rice weevil displays a set of hard, chewing mandibles at the end of its long snout, Image from the Pest and Disease Image Library, Bugwood.org
There are a few stored product insects that do not usually feed on whole kernels of grain but are still capable of chewing through thick plastic and cardboard and are considered to be top-tier penetrators in this article.
This list includes the cigarette beetle, Lasioderma serricorne, and drugstore beetle, Stegobium paniceum, who are related to woodfeeding beetles with muscular jaws capable of boring into hard wood.
Similar to their wood-boring cousins, the heads of adult cigarette beetles and drugstore beetles, face straight downward in comparison to their body. This positioning gives added leverage to chew their way through tough materials.
Another insect in this category is the red-legged ham beetle, Necrobia rufipes. While neither the larvae nor adult red-legged ham beetle have downward-facing heads, they are active and aggressive chewers, capable of working their way through relatively thick packaging materials to get to the food on the other side.
Alternatively, stored product insects that typically feed on milled, crushed or broken grains, do not have the same muscular strength or sharp mandibles that the internal grain feeders have. While this is true, they can still penetrate plastic that has slight imperfections in it. They are also capable of positioning themselves within folds in plastic or paper packaging to get enough leverage to chew through these types of outer wraps.
This list of mid-tier penetrators includes; larval stages of Indianmeal moth, Plodia interpunctella, Mediterranean flour moth, Ephestia kuehniella, almond moth, Cadra cautella, red flour beetle, Tribolium castaneum, confused flour beetle, Tribolium confusum, warehouse beetle, Trogoderma variabile, black carpet beetle, Attagenus unicolor, and other species.
Webbing clothes moth, Tineola bisselliella larvae are also a mid-range penetrator capable of chewing through thin plastic material and paper (see image below) to get to a source of wool or other animal fibers.
A Webbing Clothes Moth larva easily chews through a thin Low-Density Polyethylene (LDPE) film that is commonly used as grocery bags in the U.S.A. Photo by Patrick Kelley, Insects Limited, Inc
There is also a group of low-tier penetrators that are not so good at chewing their way through packaging materials but are good at finding ways through packaging that do not require chewing.
This group includes; saw-toothed grain beetle, Oryzaephilus surinamensis, merchant grain beetle, Oryzaephilus mercator, flat grain beetle, Cryptolestes pusillus, and rusty grain beetle, Cryptolestes ferrugineus.
This group of beetles are small and thin and can work their way through tiny openings a half-a-millimeter in diameter.
The young larvae of these and other stored product insects can enter holes even smaller than this.
Question: What are the best packaging materials that can hold food or belongings while keeping pests from penetrating at the same time? Below are some of the choices:
1. Polyethylene films having a thickness of > 10 mil work well for short- or long-term storage. This is also generally the most cost-effective alternative.
2. Polyester films work well as a barrier for insects. Polyester films are those films that use Polyethylene terephthalate or (PET).
3. Aluminized barrier films such as the brand name Marvelseal® or U.S. military specification packaging (MIL-PRF-131K-Class I) are extremely pest resistant.
4. Ceramic coated transparent barrier films are also extremely insect resistant.
Stored product insects have evolved with people to take advantage of the types of packaging materials that we use to transport or cover our food and belongings.
The same morphological design in their mouthparts that allows this set of insects to chew into and through grain husks and grain kernels, also allows them to chew through paper and plastic that is so commonly used to make up our food packages used in warehouses, retail stores and pantries, or in the paper and plastic bags that we use to cover our belongings.
Considering the excellent penetrating capabilities of stored product insects in grain and packaging and taking care to use materials that are impenetrable to this group of insects, will help us protect those foods that nourish us and our pets.
This pest management strategy will also help us preserve those things that we hold near and dear to our hearts.
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