Most of us have experienced a number of insect pests that are attracted to our food, whether at home or eating out, in pubs, restaurants or picnics. These range from flies and wasps to ants and, thankfully less common here in the UK, cockroaches. They crawl and fly to the food and are relatively easy to spot because of their size.

There are, however, a number of insect pests that are difficult to notice because they are so small, especially at the egg and early larval stages, that you may need a magnifying glass to see them. Many spend a significant part of their life inside their preferred food. These are collectively referred to as stored product insects (SPIs) or stored product pests.

The threat to food supplies

These tiny pests cause a vast amount of economic damage in the food trade worldwide, from farm through storage, transport, processing, distribution and in homes and businesses. They consume, contaminate and spoil a significant proportion of the world’s food supply, especially in developing countries where poor food storage conditions enable insects to proliferate. Insect pests are estimated to destroy around 20% of the world’s grain crop production each year in the post-harvest stages.

Stored product insects can enter the food supply chain at any point, from the field on a farm all the way to the consumer. They may get delivered with the food, walk or fly into a food storage area attracted by the smell, and hide in cracks, crevices and hidden places around where it is stored. They are then ready to infest the next batch of food brought in for storage.

A number of these pests are also associated with nesting birds and rodents, living off the detritus in and around the nests. They can then spread from the nests to plant or animal products stored in the same building or buildings nearby.

You may not notice that the food you have bought is infested, even if you eat some — check closely inside your packets of flour, seeds, grains or other dried foods. It may take a large population to build up in some food before their faeces and caste skins change the taste and smell of the food. For food processors, however, contaminated ingredients can affect product taste, change physical properties and clog production machinery, causing financial losses.

What are stored product insects?

There are three main categories of these pests:

• Beetles and weevils: these form the largest order in the animal kingdom, Coleoptera, with around 400,000 species. The adults and larvae have biting mouthparts and a relatively small number of species are adapted to feed on and breed in a range of dried plant and animal products that humans use. A few hundred species are significant pests. Most of these beetles and weevils can fly when they are adults to find new feeding and breeding sites. A few, however, have no functional wings and rely on human activity to stay near a suitable food supply.
• Moths: the order Lepidoptera has around 180,000 species of butterflies and moths, but only around 30 are significant pests of stored products. Adult moths have no biting mouth parts — they suck liquid food — but the larvae have biting mouth parts for eating solid foods and can chew through some types of packaging.  The winged adults, obviously, are the most mobile and can fly to the food sources suitable for laying their eggs.
• Mites: these are not insects, but arachnids — closely related to spiders and ticks— and the smallest of the pests at less than 1mm across when adult. They not only contaminate and taint food but can cause allergic reactions. They can infest a wide range of foods but some are beneficial, being parasites of beetles or weevils that infest food.

What foods do stored product insects eat?

Stored product insects can infest any dried and preserved foods and organic products, including grains, nuts, cereals, pasta, cheeses, preserved meats, wool and leather. They can be classified according to the plant and animal products that they infest.

The University of California Riverside and the UK Chartered Institute of Environmental Health classified insects into groups with similar habits in the way they infest stored products. Here are the categories and some of the most important pests — some feed on multiple food types and so appear in more than one category.

Whole grain pests

These are some of the most economically important pests of food globally, being able to destroy large amounts of grain in storage. Some such as the Khapra beetle is regarded an invasive species worldwide and subject to quarantine in countries such as the US and Australia:

• Granary weevil, Sitophilus granarius
• Rice weevil, Sitophilus oryzae
• Lesser grain borer, Rhyzopertha dominica
• Cadelle, Tenebroides mauritanicus
• Angoumois grain moth, Sitotroga cerealella
• Khapra beetle, Trogoderma granarium

Broken grain pests

This group of stored product insects feed on broken grains, flour, cereals and a wide range of other dried plant and processed products. They include some of the most important pests in grocery stores and homes. As there will always be some broken grains in any grain store, they can also be present in unprocessed whole grain stores and containers.

• Confused flour beetle, Tribolium confusum
• Yellow mealworm, Tenebrio molitor
• Flat grain beetle, Cryptolestes pusillus
• Siamese grain beetle, Lophocateres pusillus
• Broadnosed grain weevil, Caulophilus oryzae
• Mediterranean flour moth, Ephestia  kuehniella
• Indian meal moth, Plodia interpunctella
• Australian spider beetle, Ptinus ocellus
• Whitemarked spider beetle, Ptinus fur
• Dermestid beetles (Dermestes, Anthrenus, Trogoderma, Attagenus)  generally carrion feeders, they are common in warehouses, granaries, four mills and food packaging plants

Meat and cheese pests

Dermestes beetles are generally carrion feeders and are pests of preserved and processed meats and cheese. They can breed in carpets, hides, bird and insect nests and dead rodents — they are even used to clean animal skeletons in museums. Larvae can bore into wood or other hard materials to pupate.

• Redlegged ham beetle, Necrobia rufipes
• Larder beetle, Dermestes lardarius
• Black larder beetle, Dermestes ater
• Hide beetle, Dermestes maculatus

General feeders

These species have been found in a wide range of foods in addition to those indicated by their names, including cereals, flour, nuts, seeds, chocolate, spices, beans, tobacco, dried fruit.

• Sawtoothed grain beetIe, Oryzaephilus surinamensis
• Merchant grain beetle, Oryzaephilus mercator
• Cigarette Beetle, Lasioderma serricorne
• Biscuit beetle, drugstore beetle, Stegobium paniceum
• Tobacco moth, Ephestia elutella
• Almond moth, Cadra cautella

Dried fruit pests

These stored product insects are generally feeders of multiple food types. They are more likely to infest dried fruit if it has been stored for long periods or if it is fermenting or decaying through poor storage. Some beetles feed on mould growing on rotting foods.

• Indian meal moth, Plodia interpunctella
• Mediterranean flour moth, Anagasta kuehniella
• Almond moth, Cadra cautella
• Driedfruit moth, Vitula edmandsae serratilineella
• Stored nut moth, Aphomia gularis
• Sawtoothed grain beetle, Oryzaephilus surinamensis
• Merchant grain beetle, Oryzaephilus mercator
• Dried fruit beetle, Carpophilus hemipterus
• Tobacco beetle, Lasioderma serricorne
• Flour beetles, Tribolium species
• Rusty grain beetle, Cryptolestes ferrugineus

Nut pests

• Confused flour beetle, Tribolium confusum
• Rust red flour beetle, Tribolium castaneum
• Saw-toothed grain beetle, Oryzaephilus surinamensis
• Merchant grain beetle, Oryzaephilus mercator
• Indian meal moth, Plodia interpunctella

Legume pests — beans and peas

Bruchid beetles, such as the Bean Weevil, Acanthoscelides obtectus. This was originally a tropical species, from Central America, but has spread around the world in food shipments and is a significant pest in many countries.

Mites

Mites are widely distributed in the environment worldwide, occurring in a wide range of habitats, from soil, plant litter, under bark, as parasites of plants, birds, mammals and insects, and in nests of birds, rodents and bees. They are arachnids, not insects, which makes them more closely related to spiders, ticks and scorpions. They are the smallest of the pests when adult, at under 1 mm in length.

Several species are major pests of flour, grain, seeds, nuts, cheese and preserved meats. Some also feed on the fungi that develop on foods when they are stored in damp conditions and others are parasites of some of the beetle pests listed earlier. At least two species are also deliberately cultured on some cheeses to give a distinctive flavour.

Mites produce allergens that can affect people handling infested grain and flour and also dust and infested furnishings in the home.

• Grain or flour mite, Acarus siro — the most well-known of the mites, infesting, as its name suggests, flour and grain but also other foodstuffs. It is also used to flavour the French cheese Mimolette
• Cheese mite, Tyrophagus casei  — a  pest of cheese, grain and flour, but it is also purposely cultivated on some cheeses for the specific flavour they create, such as the German cheese Milbenkäse
• Mould mite, Tyrophagus putrescentiae — found in foods with high fat content such as seeds, nuts, cheese, ham, and also on grains and flour. It causes a skin irritation called grocer’s itch
• Driedfruit mite, Carpoglyphus Iactis – occurs on dried fruits such as figs, dates, sultanas, raisins and fermenting food products
• Food mite, furniture mite, dust mite, Glycyphagus domesticus — feeds on flour, sugar, cheese, tobacco, wheat, hay, mould, and is found in bee and bird nests.

Signs of stored product insects

It is important to conduct detailed inspections of stored products and storage areas to identify an infestation as early as possible. Examine deliveries on arrival and make regular checks on food that has been in storage for a while — make sure suppliers and shipping or transport agents also have a regime of inspection and monitoring.

The common signs of a stored product insect infestation are:

• A product shows signs of damage: for example grains, seeds, nuts have small bore holes
• Live or dead insects in the food: adult insects, eggs, larvae, frass, pupae or silken webbing in the food or packaging
• Food spillages: contain insect adults, larvae, pupae or silken webbing
• Holes in packaging: these can be very small — larvae or webbing on the outside of packets or bags
• Cupboards and shelves, in the storage area, or machinery have adults, larvae, frass, pupae or silken webbing
• Beams and window sills: where food is stored these have larvae, pupae or silken webbing
• Caught in traps: stored product insects are caught in insects traps or electric fly killers
• Infested machinery: adults, larvae, pupae or silken webbing are in crevices or hidden spaces