While primary pests of stored product attack undamaged, whole grains and develop inside the grains, secondary pests feed externally, and attack broken, damaged grain and powdered products. They may infest stored food following the primary pests. These secondary pests include the Indian meal moth and a variety of beetle species, such as the sawtoothed grain beetle, cigarette beetle, drugstore beetle, the red flour and confused flour beetles.

Indian meal moth

(Plodia interpunctella)

Description: Adult moths are very good flyers but erratic in their movement, with no apparent direct line of flight. Adults are 12-13 mm long with forewings that fold over the abdomen without crossing in the middle. Indian meal moths also have three black stripes on each forewing. The base of the wing has a grey coloration and the distant part of the forewing has a brownish- red coloration. Larvae spin silk and cause damage to products not even consumed by them as a result. Pupae are light brown with white silk around its perimeter and attached to various objects away from the food source. Eggs will be almost unnoticeable in diameter (0.5 mm) and grey or white in coloration. Larvae are small and white with a brownish-red head capsule. Larvae are also highly mobile with small walking legs. Pupae are brownish in color with white silk surrounding them and they can be located next to or several feet away from the larval food source.

Host: Larvae are generalists and will feed on a wide variety of stored products from gains and cereal to dogfood. The larvae create “nests” by spinning silk which traps their fecal pellets and castings which results in the majority of damage to stored products. Larvae feed externally on their food rather than burrowing into it and are incapable of chewing threw whole grain. Adults do not feed.

Life cycle: Adults can live for up to 25 days. Adult females can lay up to 400 eggs in their lifetime. Eggs are deposited on their food source and hatch around seven days after. Larvae pupate after 6 to 8 weeks. Pupation can take place close to the food source or several feet away due to their larval mobility and the need to be distanced from other feeding larvae. The pupal stage can last up to 20 days where, after eclosion, the adults will mate three days later. Developmental time is temperature and humidity dependent.

Sawtoothed grain beetle

(Oryzaephilus surinamensis)

Description: The beetle is about 2.5 mm long as adults while being flat and slender. The species has a characteristic saw-like series of spines running down either side of their thorax. Adults head capsules are slightly more distanced from their thorax when compared to merchant grain beetle. They also have larger eyes and a more triangular head than the merchant grain beetle. Sawtoothed grain borer coloration is brownish-red as adults with small white colored larvae and eggs that are never seen outside the kernel. Adults have very developed hind wings, but they are not observed to fly.

Host: Sawtoothed grain beetles are affiliated with breakfast cereal in urban settings. They are generalists and feed on any stored foods of vegetable origin which include cereal, grain, chocolate, shelved drugs, and tobacco products. They can quickly become a problem for any homeowner and are commonly an issue for grocery stores.

Life cycle: Adults can lay hundreds of eggs in their life time and have been recorded living for over 3 years. Females chew a hole through the kernel, deposit an egg, then re-seal the egg in its food source using ovipositional fluid. Dependent on temperature and humidity, after oviposition, eggs hatch in 3-5 days and the larvae can take about 8-10 weeks to begin the pupal state and an additional 1-3 weeks to emerge as adults.

Merchant grain beetle

(Oryzaephilus mercator)

Description: Appearance of the merchant grain beetle are similar to the sawtoothed grain beetles. However, merchant grain beetles are slightly shorter at 2.5 mm in length. They have a saw-like series of spines along either side of their thorax. The head, thorax, and abdomen are connected closely with limited spacing unlike sawtoothed grain beetles. The larvae and eggs are also yellowish-white in color and live internally in their food source. Adults can fly with fully developed hind wings.

Host: Merchant grain beetles are most commonly affiliated with oilseed and oilseed products. Like sawtoothed grain beetles, they will feed on various plant-based products like cereal, grains, chocolate, shelved drugs, dried fruits, and tobacco products. This species prefers products with higher fat content. However, adults and larvae are not able to chew threw whole grains.

Life cycle: Females can lay over 250 eggs. Larvae feed thought the food source and are not encapsulated in it like sawtoothed grain beetle larvae. When the larvae are mature, they use the surrounding food source to weave a protective casing while they are in pupation. Total time from egg to adult can take up to 50 days depending on environmental conditions.

Cigarette beetle

(Lasioderma serricorne)

Description: Cigarette beetles are deep brown in color and oval shaped. They are roughly 3 mm long and have the ability to fly. It does not have any abnormal dents, spots, or blemishes on its elytra or thorax like other closely related species Cigarette beetle adult. The elytra do not possess grooves. Antennae are light brown with 11 antennal segments that are sawlike. The larvae are grublike, white in color, mobile, and will forage for food after hatching.

Host: As their name suggests, cigarette beetles target tobacco and tobacco products. Their larvae feed on tobacco after it has been cured. The larvae will target both the dried leaves as well as tobacco products. They also target other plant based dried products like most stored product pests. However, they avoid targeting certain stored products like whole grains because they are unable to penetrate the kernel unless it is damaged. Adults do not feed.

Life cycle: Adult females can lay about 100 eggs in their lifetime. Females will oviposit on tobacco leaves and other stored products. The egg stage can be 4-10 days with larval stage lasting up to 50 days. They remain as pupae for about 12 days. On average, their total life cycle from egg to adult is 70 days but, under favorable conditions, can be as early as 26 days. Because adults do not feed, they live roughly 2-6 weeks as adults.

Drugstore beetle

(Stegobium paniceum)

Description: Misnamed as the biscuit weevil, drugstore beetles are not a weevil and, instead, have a generic beetle appearance. They are 3.5 mm in length and oval shaped with a semi-triangular thorax that covers their heads when viewed from above. They are similar in appearance to the cigarette beetle, but can be distinguished by the grooves on elytra and antennal shape. Unlike cigarette beetles, the elytra of drugstore beetles possess distinct grooves, and the antennae have 3-segmented clubs at the tips. Their elytra are covered in small light brown hairs giving it a velcro looking texture. Eggs are white in color. Drugstore beetle larvae also appear to be grub like, mobile, and white. However, drugstore beetle larvae are not as hairy as cigarette beetle larvae.

Host: Drugstore beetles target mainly plant based organic products like herbs, bread, tobacco, and other dried plant products. Although the species can chew through thin packaging, it is considered a secondary pest of stored products because the larvae are unable to chew threw whole wheat kernels.

Life cycle: Adult females can lay 100 eggs in their 25-day lifespan. Under optimal conditions, which were 30°C and 60-90% relative humidity, larvae can complete development from egg to adult in 40 days. Developmental time is dependent on humidity and temperature.

Red flour beetle

(Tribolium castaneum)

Description: Adults are up to 4 mm in length and yellowish- brown in color. General appearance and color are very similar to the confused flour beetle. Slight morphological differences are present on their antennae and thorax. The antennae have 3-segmented clubs in the end. The thorax of adults has slightly curved sides viewed from above. Eggs are solid white but are typically covered in surrounding material via a glue-like coating. Larvae are light yellow with darker brown head capsules and abdominal tips. Larvae are mobile and almost identical to Confused flour beetle larvae. Adults of the species release defensive chemicals that deter predators and can cause irritation to human skin.

Host: Known as one of the two most common secondary pests for plant derived products and commodities in the world, red flour beetles target stored grains and are a model organism for ethological and food safety programs. Like other stored product pests, red flour beetles are generalists and will target a wide variety of plant based stored products. Larvae and adults are not able to penetrate whole grain kernels.

Life cycle: With a lifespan of 3 years, adult females can lay up to 500 eggs in their lifetime. Eggs can be deposited directly onto food material or placed within damaged packaging and opened containers. Confused flour beetle eggs hatch 3-7 days after being deposited. Larvae will chew into grain and normally feed inside the food source but can be found foraging on top of it. When they pupate, they remain solitary and unprotected by their food source, unlike confused flour beetle eggs. Total time from egg to adult is around 25 days but developmental time is dependent on temperature and humidity. Minimum and maximum temperature for development to take place is 20-40°C.

Confused flour beetle

(Tribolium confusum)

Description: Adults are up to 4 mm in length and yellowish- brown in color. General appearance and color are very similar to the red flour beetle. Slight morphological differences are present on their antennae and thorax. The confused flour beetles have gradual club-like antennae. The thorax of adults is rectangular when viewed from above, and the sides of thorax are more parallel compared to red flour beetles. Eggs are solid white but are typically covered in surrounding material via a glue-like coating. Larvae are light yellow with darker brown head capsules and abdominal tips. Larvae are mobile and almost identical to red flour beetle larvae. Adults of this species also release defensive chemicals that deter predators and can cause irritation to human skin.

Host: Confused flour beetles and red flour beetles are the two most common secondary pests of stored plant products in the world. Confused flour beetles primary target are stored grains. Like other stored product pests, confused flower beetles are generalists and will target a wide variety of plant based stored products. Larvae and adults are not able to penetrate whole grain kernels,.

Lifecycle: Like red flour beetles, the confused flower beetle has a lifespan of 3 years and adult females can lay up to 500 eggs. Eggs are deposited directly onto food material or deposited around it. Confused flour beetle eggs hatch 3-7 days after being deposited. Larvae will chew through grain and normally feed inside it but can be found foraging on top of food or searching for alternate food sources. When they pupate, they remain solitary and unprotected by their food source, unlike confused flour beetle eggs. Total time from egg to adult is around 25 days but developmental time is dependent on temperature and humidity. Minimum and maximum temperature for development to take place is 20-40°C.

References

Abdelghany, A. Y., Awadalla, S.S., Abdel-Baky, N. F., El-Syrafi, H. A., Fields, P. G. (2010). Effect of high and low temperatures on the drugstore beetle (Coleoptera: Anobiidae). Journal of Economic Entomology. 103(5): 1909-1914.

Back, E. A., Cotton, R. T. (1926). Biology of the saw-toothed grain beetle, Oryzaephilus surinamensis Linne. Journal of Agricultural Research. 33(5): 435-450.

David, M. H., Mills, R. B. (1974). Development and oviposition of ahasverus advena (waltl) (coleoptera, silvanidae) on seven speciesof fungi. Journal of Stored Product Research. 10: 17-22.

Devi, M. B., Devi, N. V. (2015). Biology of rust-red flour beetle, Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae). Biological Forum.7(1): 12-15.

Hamlin, J. C., Phillips, M. E. (1931). Biology of the Indian-meal moth on dried fruits in California US Dept. of Agriculture. 242: 1-4.

Howe, R. W. (1956). The biology of the two common storage species of Oryzaephilus (Coleoptera, Cucujidae). Annals of Applied Biology. 44(2): 341-355.

Thomas E., Powell, J. (1931). An ecological study of the tobacco beetle, Lasioderma serricorne Fabr., with special reference to its life history and control. Ecological Monographs. 1(3): 333–393.

Steven Richardson is a graduate student, and Qian "Karen" Sun is an assistant professor in the Department of Entomology.

Indian meal moth adult. Photo by Whitney Cranshaw, Colorado State University, Bugwood.org

Sawtoothed grain beetle adult. Head capsule is slightly more distanced from their thorax compared to merchant grain beetle. Photo by Pest and Diseases Image Library, Bugwood.org

Merchant grain beetle adult. The head, thorax, and abdomen are connected closely with limited spacing unlike sawtoothed grain beetles. Photo by Pest and Diseases Image Library, Bugwood.org

Cigarette beetle adult. The elytra (wings) are without grooves and antennal segments are sawlike. Photo by Pest and Diseases Image Library, Bugwood.org

Drugstore beetle adults. The elytra (wings) possess distinct grooves and the antennae have 3-segmented clubs at the tip. Photo by Pest and Diseases Image Library, Bugwood.org

Red flour beetle adult. The thorax has slightly curved sides, and the antennae have 3-segmented clubs. Photo by Emilie Bess, USDA APHIS PPQ, Bugwood.org

Confused flour beetle adult. The sides of thorax are more parallel compared to the red flour beetle, and the antennae are gradually club-like. Photo by Pest and Diseases Image Library, Bugwood.org