Canadian Grain Commission
Symbol of the Government of Canada

Protection of farm-stored grains, oilseeds and pulses from insects, mites and moulds



6. Identifying the organisms

Common stored-product pests

Of more than 100 species of stored-product insects and mites found across Canada, only a few cause serious damage; the others are fungus feeders, scavengers, predators and parasites.

Beetles and moths, the most-common stored-product insect pests, have four life stages: egg, larva, pupa and adult (Fig. 10). Booklice (psocids) and mites have only egg, nymph and adult stages.

Life cycles of stored-product insects: A, a beetle and B, a moth

Fig. 10 Life cycles of stored-product insects: A, a beetle and B, a moth.

Egg

The eggs may be laid either in the crevices of kernels or in the dust and refuse within bins. Some species, such as granary weevils, lay their eggs inside kernels.

Larva

The larva is the only stage during which the insect grows. It consumes several times its own weight in food, and as the larval skin cannot stretch, it periodically moults allowing it to increase in size. Cast-off skins found in grains, oilseeds and their products indicate that insects are, or were, present.

Pupa

The pupa, which forms after the last larval molt, does not feed. In some species, the pupa is enclosed in a cell, or cocoon, constructed by the larva. During the pupal stage, the insect undergoes extreme internal and external changes that lead to the development of the adult.

Adult

Adults of stored-product insects are between 0.1 and 1.7 cm long. They have three pairs of legs and their bodies are divided into three parts: head, thorax and abdomen. The head includes the mouthparts and sense organs; the thorax bears the legs and wings; and the abdomen contains the reproductive organs. Adults move in the spaces between kernels and can penetrate deeply into a bulk of grains or oilseeds, with the exception of moths and spider beetles. Some stored-product insects can fly and are widely distributed. Beetles have poorly developed wings and some species are unable to fly, although the rusty grain beetle, the red flour beetle and the lesser grain borer fly well.

Beetles

Table 2 - Stored-product beetles found in Canada - Part 1
Common Name Scientific Name Adult colour Adult length (mm) Antennae Can climb glass Can fly
American black flour beetle Tribolium audax dark brown, black 3-4 clubbed no no
Cadelle Tenebroides mauritanicus Dark brown, black 6-10 clubbed no yes
Confused flour beetle Tribolium confusum reddish brown 2-4 clubbed no no
European black flour beetle Tribolium madens Dark brown, black 4-5 clubbed no yes
Flat grain beetle Cryptolestes pusillus reddish brown 2 thread no yes
Foreign grain beetle Ahasverus
advena
Brown 2 clubbed yes yes
Granary weevil Sitophilus granarius Dark brown, black 3-4 clubbed yes no
Hairy spider beetle Ptinus villiger Brown 2-4 thread yes no
Lesser grain borer Rhyzopertha dominca dark brown 2-3 clubbed no yes
Psocids Lepinotus reticulatus
Liposcelis bostrychophilus
light brown 1 thread yes yes
Red flour beetle Tribolium castaneum reddish brown 2-4 clubbed no yes
Rice weevil Sitophilus oryzae Dark brown, black 3-4 clubbed yes yes
Rusty grain beetle Cryptolestes ferrugineus reddish brown 2 thread no yes
Sawtoothed grain beetle Oryzaephilus surinamensis Brown 3 clubbed yes yes
Yellow meal worm Tenebrio molitor brown, black 12-17 thread no rarely
Table 2 - Stored-product beetles found in Canada - Part 2
Common Name Scientific Name Shortest egg to adult (days) Food Occurance in Eastern Canada Occurance in Western Canada
spotty = consistent but-isolated local infestations
rare = found only occasionally
American black flour beetle Tribolium audax 25 flour spotty rare
Cadelle Tenebroides mauritanicus 70 broken cereals, damp grain spotty spotty
Confused flour beetle Tribolium confusum 20 Flour common common
European black flour beetle Tribolium madens 25 Flour spotty rare
Flat grain beetle Cryptolestes pusillus 22 broken cereals spotty spotty
Foreign grain beetle Ahasverus advena 21 Mould common common
Granary weevil Sitophilus granarius 28 cereal seed spotty rare
Hairy spider beetle Ptinus villiger 58 flour, feed spotty spotty
Lesser grain borer Rhyzopertha dominca 25 cereal seed rare rare
Psocids Lepinotus reticulatus
Liposcelis bostrychophilus
21 mould common common
Red flour beetle Tribolium castaneum 20 broken cereals, flour common common
Rice weevil Sitophilus oryzae 24 cereal seed spotty spotty
Rusty grain beetle Cryptolestes ferrugineus 21 broken cereals common common
Sawtoothed grain beetle Oryzaephilus surinamensis 20 broken cereals spotty spotty
Yellow meal worm Tenebrio molitor 120 mouldy grain common common

Stored-product beetles often appear similar but have differing behaviour patterns and status as pests. It is important to determine which species are present before taking remedial action. A detailed identification guide is now available (Bousquet 1990) to help determine which species are present. The characteristic features of the main beetle species occurring on stored grains and oilseed crops in Canada are as follows:

Rusty grain beetle

This beetle (Plate 1a, b) is the most serious pest of stored grain in most regions of Canada. It usually feeds on the germ (embryo) part of a whole seed. Heavy infestations cause grain to spoil and heat. The adult is a flat, rectangular, shiny, reddish-brown beetle, 0.2 cm long and has long, bead-shaped antennae that project forward in a “V”. It moves rapidly in warm grain and can fly when the air temperature is above 23°C. Eggs are laid in the crevices of kernels and in grain dust. The tiny larvae penetrate and feed on the germ of damaged kernels. Eggs become adults in wheat in about 21 days at 14.5% moisture content and 31°C.

Wheat kernels infested by rusty grain beetles

Plate Ia - Wheat kernels infested by rusty grain beetles.

Life stages of the rusty grain beetle (left to right): eggs, larva, pupa and adult

Plate Ib - Life stages of the rusty grain beetle (left to right): eggs, larva, pupa and adult.

Flat grain beetle

This insect is similar in appearance and feeding habits to the rusty grain beetle except that the males have longer antennae. It is an important pest of stored grain in the northern United States and is now appearing in grain bins in southern parts of the Canadian prairies.

Red flour beetle

This pest (Plate IIc, d) develops on stored grains and oilseeds on farms and in primary elevators throughout the Prairie Provinces and most of Canada. The adult is reddish brown and 0.4 cm long. Larvae and adults feed on broken kernels. Complete development from egg to adult occurs in about 28 days under optimal conditions of 31°C and 15% moisture content. Slower development occurs at moisture contents as low as 8%. Adults fly in warm weather or may be blown by the wind into farmhouses or other buildings.

Wheat kernels infested by red flour beetle adults.

Plate IIc - Wheat kernels infested by red flour beetle adults.

Life stages of the red flour beetle (left  to right): eggs, larva, pupa and adult.

Plate IId - Life stages of the red flour beetle (left to right): eggs, larva, pupa and adult.

Confused flour beetle

The adult (Plate IIe) resembles that of the red flour beetle and is difficult to distinguish without a microscope or magnifying glass. Larvae and adults feed on flour, animal feed and other ground material. Unlike the red flour beetle, the confused flour beetle is more common in flour mills than elsewhere, and the adults do not fly.

Life stages of the confused flour beetle (left to right): eggs, larva, pupa and adult.

Plate IIe - Life stages of the confused flour beetle (left to right): eggs, larva, pupa and adult.

American black flour beetle

This beetle is similar to, and larger than the red flour beetle, but black in color. It is commonly found in empty granaries but rarely infests farm-stored grains and oilseeds in large number.

Fungus beetles

These pests usually infest tough or damp grains and oilseeds and feed on associated dust and moulds. Dry seed bulks stored next to tough or damp seed bulks may also become infested. The foreign grain beetle, the square-nosed fungus beetle, and the sigmoid fungus beetle are the most common fungus-feeding insects found in stored grain and oilseed crops. Because certain species of fungus beetles resemble the rusty grain beetle and are about the same size, apply chemical control measures only after the insects are correctly identified. The foreign grain beetle is similar to the rusty grain beetle but is able to climb up glass whereas the rusty grain beetle cannot.

Fungus beetles in stored grains and oilseeds are cause for as much concern as are rusty grain beetles, because they indicate that high moisture and moulds are present and that the crop may be going out of condition. The grains or oilseeds must be dried to break up tough or damp pockets. As fumigation will not stop spoilage by moulds or heating, take measures to move the bulk immediately, or it may spoil resulting in significant losses.

Sawtoothed grain beetle

These beetles (Plate Ic) are more common in oats than in wheat, barley or canola, particularly in southern Ontario and Quebec. The adult is brown, is about 0.3 cm long, and has six tooth-like projections on each side of the thorax. In warm grain it takes about 22 days to develop from egg to adult under optimal conditions of 31 to 34°C and 14 to 15% moisture content.

Granary weevil

This weevil (Plate Ie) is one of the most destructive pests of stored grain in the world. It is scarce on the prairies but occurs in Ontario and grain terminals in Vancouver. The adults have a distinctive snout, with which they bore into grain kernels. The female deposits a single egg in a hole in each kernel and then seals the opening with a gelatinous plug. The larvae feed on the endosperm and complete their development within the kernel. The pupae develop into adults that chew holes in the side of the kernels as they emerge. Development from egg to adult takes 25 to 35 days under optimal conditions of 26 to 30°C and 14% moisture content. The granary weevil adult is about 0.3-0.4 cm long and cannot fly. When disturbed, they fold their legs under their body and appear to be dead.

Life stages of the sawtoothed grain beetle (left to right): egg, larva, pupa and adult.

Plate Ic - Life stages of the sawtoothed grain beetle (left to right): egg, larva, pupa and adult.

Life stages of the granary weevil (left to right): egg, larva, pupa and adult.

Plate Ie - Life stages of the granary weevil (left to right): egg, larva, pupa and adult.

Rice weevil

This weevil (Plate If) has been found in southwestern Ontario storage and in some prairie elevators in recent years. It is 0.2 to 0.4 cm long and has four distinct reddish orange spots on the wing covers, which are folded over the abdomen. It completes development from egg to adult in 28 days at 30°C and 14% moisture content. Adult rice weevils can fly, and attack a wide range of cereals other than rice; larvae develop and pupate within the kernel.

Life stages of the rice weevil (left to right): egg, larva, pupa and adult.

Plate If - Life stages of the rice weevil (left to right): egg, larva, pupa and adult.

Lesser grain borer

This insect is a major pest of wheat in the plains of the USA. Adults are dark brown, cylindrical, 0.2 to 0.3 cm in length and the head is invisible when viewed from above. Adults are strong flyers. Larvae bore into the seed. In warm grain development takes 25 to 30 days. The lesser grain borer is found occasionally in grain, and has been caught in pheromone-baited flight traps across the Canadian prairies and at terminal elevators in Vancouver and Thunder Bay. However, this insect is rare in Canadian stored grain and it is unlikely that it can survive the entire winter in farm bins in Western Canada.

Hairy spider beetle

This beetle is mainly a pest of wheat flour and animal feeds but may also infest stored grain near the surface. Adults and larvae have strong jaws, which they use to chew large, irregular holes in the endosperm of kernels. The adult is 0.35 cm long and has long, spiderlike legs and long, thin antennae. This beetle has only one generation a year. Three or four larvae often cement five to eight kernels together to form a cluster, where they feed and grow for up to 5 months; then each constructs its own pupal cell, from which the adult emerges.

Yellow mealworm

These insects (Plate IIa) are the largest found in stored grain. They are not common pests on farms. They first infest animal feeds and then move into stored grain that is going out of condition. The adults are black beetles about 1.5 cm long; the larvae are yellow and 0.2 to 2.8 cm long. Yellow mealworms prefer dark, damp places in a granary or a feed bin. The adults live for several months and the larvae may take 1 to 2 years to change into pupae under harsh conditions. Because of their relatively large size, they are easily visible and often appear to be more numerous than they actually are. Their presence indicates poor storage and sanitation conditions.

Life stages of the yellow mealworm (left to right): eggs, larva, pupa and adult.

Plate IIa - Life stages of the yellow mealworm (left to right): eggs, larva, pupa and adult.

Psocids (booklice)

These insects are slightly larger than grain mites. The adult is soft-bodied and about 1.0 mm long. It has a large head and long antennae, and some species have wings and may be confused with small flies (Fig. 11C). The female lays about 100 eggs in 3 weeks, which develop into adults during the summer. The egg develops through nymph to adult in about 21 days at 27°C and 13% moisture content; some adults can live for 51 days without feeding. In most years psocid cause no major problems, although they can feed on damaged kernels and are found in tough or damp grain. Occasionally, they occur in large numbers in widespread areas without any warning but do not cause serious damage to the stored crops. They are usually found with other insects or mites that are more serious pests of stored grains and oilseeds, often feeding on their eggs.

Moths

These pests are common in central Canada and on the east and west coasts. Adult moths do not feed, but their larvae have strong mouthparts and cause extensive surface damage to stored grain. Low winter temperatures usually control moth infestations, which are confined mainly to the surface layers of tough or damp grains that may be heating.

Table 3 - Stored-product moths found in Canada - Part 1
Common Name Scientific Name Adult colour Adult length (mm) Antennae Can climb glass Can fly
Brown house moth Hofmannophila pseudospretella brown 8-11 thread yes larvae yes
Indianmeal moth Plodia interpunctella cream and brown 8-10 thread yes larvae yes
Meal moth Pyralis farinalis brown and tan 15-20 thread yes larvae yes
Mediterranean flour moth Ephestia kuehniella gray and black 10-15 thread yes larvae yes
Whiteshouldered house moth Endrosis sarcitrella white 8-11 thread yes larvae yes
Table 3 - Stored-product moths found in Canada - Part 2
Common Name Scientific Name Shortest egg to adult (days) Food Occurance in Eastern Canada Occurance in Western Canada
Brown house moth Hofmannophila pseudospretella 365 grain products, dry goods yes BC ports only
Indianmeal moth Plodia interpunctella 25 grain, nuts, dried fruits, dry goods common common
Meal moth Pyralis farinalis 42 mouldy grains, dry goods ports ports
Mediterranean flour moth Ephestia kuehniella 30 flour rare rare
Whiteshouldered house moth Endrosis sarcitrella 24 grain products,  dry goods ports ports

Indianmeal moth

This moth (Plate Id) is common in central Canada primarily on corn and processed feeds and foods, and throughout the country in warehouses and stores.

Meal moth

This moth (Plate IIIb, Plate IIIc) is moderately cold-hardy and can overwinter and thrive during warm months in unheated farm granaries across the Prairie Provinces. It usually occurs in patches of moist, mouldy grain. The larvae are cream-colored, have black heads, and are about 2-cm long when full-grown. They produce a silklike substance that webs the kernels together in clumps. The moth has a wingspread of 2.5 cm. The fore wings are light brown, with dark brown patches at the bases and tips. Each wing has two wavy, white stripes. The life cycle takes about 2 months to complete in summer.

Brown house moth, whiteshouldered house moth, and Mediterranean flour moth, these moths (Plate IIIa) commonly occur in grain terminals on the east and west coasts.

Life stages of the Indianmeal moth (left to right): egg, larva, pupa and adult.

Plate Id - Life stages of the Indianmeal moth (left to right): egg, larva, pupa and adult.

Mediterranean flour moth

Plate IIIa - Mediterranean flour moth

Meal moth (adult)

Plate IIIb - Meal moth (adult)

Meal moth (larva)

Plate IIIc - Meal moth (larva)

Mites

Table 4 - Stored-product mites found in Canada - Part 1
Common Name Scientific Name Adult colour Adult length (mm) Antennae Can climb glass Can fly
cannibal mite Cheyletus eruditus white 0.4-0.6 - yes no
glossy grain mite Tarsonemus granarius orange, yellow 0.1-0.2 - yes no
grain mite Acarus siro white, tan 0.3-0.6 - yes no
longhaired mite Lepidoglyphus destructor white 0.3-0.5 - yes no
mould mite Tyrophagus putrescentiae white 0.3-0.5 - yes no
Table 4 - Stored-product mites found in Canada - Part 2
Common Name Scientific Name Shortest egg to adult (days) Food Occurance in Eastern Canada Occurance in Western Canada
cannibal mite Cheyletus eruditus 19 mites, insect eggs common common
glossy grain mite Tarsonemus granarius 5 mould common common
grain mite Acarus siro 14 cereal germ common common
longhaired mite Lepidoglyphus destructor 19 cereal, mould common Common
mould mite Tyrophagus putrescentiae 9 mould common common

Mites are the smallest of the stored-product pests. They are common in grain stored at 14-17% moisture content but, because of their microscopic size, often go unnoticed. Mites, belonging to the same class as spiders and centipedes, are fragile creatures that are hard to see with the naked eye (Fig. 11, Fig. 12). Unlike an adult insect, which has a distinct head, thorax, abdomen, and six legs, an adult mite has a saclike body with eight legs; a larva has six legs. Mites are cold-hardy; most feed on broken grain, weed seeds, dockage and moulds. They are therefore well adapted for infesting stored products. Some mites, such as the cannibal mite, feed on their own members, other mites or insect eggs. They breed in tough and damp pockets of cereals and canola. About eight kinds of mites are common in farm granaries and elevators. Some give a strong, minty odour to infested grains and oilseeds. Their life cycle consists of the egg, a six-legged larva, two or three eight-legged nymphal stages, and the eight-legged adult. Some mites change into a nonfeeding developmental stage called a hypopus during which they become resistant to low winter temperatures, drying, starvation and most fumigants; they may be mobile or inactive. This stage can last for prolonged periods until developmental conditions improve.

Fig. 11 Scanning electron microscope views:

Cannibal mite

A – Cannibal mite with large holding mouthparts

Grain mite

B – Grain mite

Winged psocid

C – Winged psocid with long antennae.

Fig. 12 Some major stored product mites as viewed by scanning electron microscope:

The grain mite   Warty grain mite

A – The grain mite (left); B – Warty grain mite (right);

Mould mite   Longhaired mite

C – Mould mite (left); D – Longhaired mite (right);

glossy grain mite female viewed from above   glossy grain mite female viewed from the side

E – Glossy grain mite female viewed from above (left);

F – Glossy grain mite female viewed from the side (right)

Grain mite

This mite (Fig. 11B, Fig. 12A) attacks the germ (embryo) of seeds, which reduces germination, and spreads fungi (moulds), which are also eaten. Heavily infested grain becomes tainted and unpalatable as animal feed. In some cases, dairy cattle and other farm animals develop gastric disorders and other symptoms after eating mite-infested feed. Adults are 0.3 to 0.6 mm long and females are larger than males. This mite is pearly white to yellow brown and has a smooth, glistening body with four long hairs arising from the rear end. Grain mite populations can increase up to sevenfold in 1 week in stored grains and oilseeds, particularly during the fall. Adult females can lay about 500 eggs during a lifespan of 42 days. The grain mite can develop from egg to adult in 14 days at 20°C and 14% moisture content. Adults and all immature stages except the nonfeeding developmental stage die in about 1 week when exposed to -18°C. Eggs can survive exposure to -10°C for about 12 days or 0°C for 2 to 3 months.

Longhaired mite

This species (Fig. 12D) is the most common stored-product mite. It is cold-hardy and can live in both straight-grade and tough grains and oilseeds. It moves rapidly with a jerky gait and feeds on broken grain, grain dust and fungi. The adult is white and about 0.3-0.5 mm long and has many stiff hairs that are longer than its body. In farm granaries, chronic infestations of this mite generally occur between June and November. It can survive for more than 7 days at -18°C.

Cannibal mite

As the name suggests, this mite (Fig. 11A) feeds on its own kind, and also on the grain mite, the longhaired mite, and insect eggs and larvae. Because cannibal mites are not abundant enough to eliminate the mite pests that damage grains or oilseeds they are not very useful for biological control of grain-feeding mites. They have a diamond-shaped, white body with a chalky white line running the length of the body, pincerlike grasping appendages near the mouth, and long legs. They are 0.4 to 0.6 mm long. Cannibal mites are active in bulk grain in all seasons, usually in low numbers. In most tough grains and oilseeds they can breed between 12 and 27°C.

Glossy grain mite

This mite (Fig. 12E, Fig. 12F) is a common fungus-feeder found in aging farm-stored grains and oilseeds. It develops from the egg to adult stage in 7 days under optimal conditions of 30°C and 17% moisture content. The mite feeds on certain moulds so its presence indicates that grain is becoming mouldy and going out of condition. The adult is clear white and less than 0.2 mm long. It can live for 17 days at 30°C and 90% relative humidity.

Storage fungi (moulds)

These organisms, occurring mainly as spores in the soil and on decaying plant material, contaminate grains and oilseeds with low numbers of spores during harvesting.

Storage fungi are usually inactive at low grain-moisture levels. However, when the moisture is higher, as in tough, damp or accidentally wetted grain, the spores germinate. Several species of Aspergillus and Penicillium are found on grains. Each fungal species requires a specific moisture and temperature level for germination and development, and develops in a definite sequence. The first fungus to develop breaks down nutrients in the seed through its enzymatic activity and produces moisture, which allows other fungi to germinate in their turn.

Storage fungi on grains and oilseeds affect their quality by causing heating and spoilage, packing or caking, reduced germination, and production of off-odours and mycotoxins. For further information on moulds and their effects on stored products, see Storage of cereal grains and their products, 4th Edition, Chapter 9 (Sauer 1992). Health hazards to humans and animals from the dust-like spores include “farmer’s lung” and allergies.

Mycotoxins

Mycotoxins are naturally-occurring fungal products which are poisonous when eaten or inhaled. These toxins occur in grain-based feeds, foods and dusts. Aspergillus and Penicillium moulds growing on stored cereals will start producing mycotoxins after about eight weeks of favorable temperature and moisture conditions. Mycotoxins can occur anywhere in Canada where grain is stored.

Although highly toxic in the pure form, mycotoxins are not usually present in dry grain, unless it becomes wet. When present at low concentrations, usually at parts-per-million, they are quickly detectable with modern test kits using the enzyme immunoassay principle. The health of farm animals can be impaired by mycotoxins in feed at the parts-per-million level or less, with livestock showing reduced productivity and increased mortality. Producers suspecting mycotoxin poisoning should save a sample of the feed, and consult their local veterinarian.

Mycotoxins usually develop when stored cereal grains become contaminated with Aspergillus and Penicillium moulds, following faulty storage or accidental dampening from seepage and condensation. In storage tests on damp grain, specific mycotoxins have been identified. Ochratoxin and citrinin are generally found in cereals contaminated with Penicillium, and sterigmatocystin is found during heavy growth of Aspergillus versicolor. The risk of these toxins forming at levels high enough to harm livestock depends on the particular crop:

  • low risk: oats, hard red spring wheat, medium-protein wheat, 2-row barley
  • moderate risk: corn, 6-row barley, hulless barley
  • high risk: amber durum wheat.

Although the aflatoxins are well-known contaminants of grains and oilseeds from tropical countries and the USA, surveys of Canadian stored crops indicate that these toxins are not present.

During years of high mid-summer rainfall, additional mycotoxin problems can develop before harvest and storage. Fungi of the Fusarium type can infect standing wheat and barley and produce white or pink shriveled kernels, which are characteristic of fusarium head blight disease. The most common mycotoxin is deoxynivalenol (DON), but more poisonous mycotoxins from Fusarium may also be present. DON is quickly detectable by modern test kits using the enzyme immunoassay principle. Swine are affected by parts-per-million levels of DON in their feed, but other livestock show considerable resistance. Fusarium also reduces yield and grade of cereals and adversely affects the baking quality of wheat and the malting quality of barley, but requires very high moisture for growth and will generally not increase in storage.

For further information, see Mycotoxins in agriculture and food safety (Sinha and Bhatnagar 1998).