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Spider Beetles

General Facts

Named for their cursory similarities to spiders, spider beetles are opportunistic scavenger pests of stored products. They tend to infest products that are old and moisture compromised with mould growth. They belong to the insect family Ptinidae, along with approximately 450 other species of beetles. Uniquely, adult spider beetles are known to remain active in freezing temperatures, which makes them an especially prevalent problem in the northern United States and Canada. In fact, about 20 different species of spider beetles are regular nuisances in Canada, with British Columbia having the highest concentration of spider beetles.

Appearance / Identification

image of american spider beetle
Though the different species of spider beetles vary in appearance, most bear a resemblance to spiders upon first glance thanks to their globular abdomens and the way their heads remain hidden from view when observed from above. Most species range in size from 2 to 5 mm and appear oval or cylindrical in shape. Each spider beetle has six legs and a pair of long antennae. The insects vary in colour from reddish-brown or black to pale yellow or cream, with distinctive markings that depend on the specific species.

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Spider beetles are largely cosmopolitan in distribution. Capable of remaining active in the cold and reproducing even when temperatures drop as low as 10º C, the insects live throughout the world, wherever human civilizations exist. Due to their scavenging nature and preferred food sources, spider beetles often infiltrate warehouses, mills, grain storage facilities, museums, and attics with rodent or bird nests. Additionally, the beetles exhibit a preference for moisture.


A noted stored product pest, spider beetles mainly consume grains. Common food sources include almonds, animal skin, beans, books, cereals, chocolate powders, corn meal, dates, dead insects, dried fruits, mushrooms, dried soups, excrement, feathers, figs, fish meal, flour, ginger, hair, leather, corn, nutmeg, older woods, paprika, rye and rye bread, seeds, silk, textile fabrics, wheat, and wool.

Life Cycle / Reproduction

Most species of spider beetles follow a similar reproductive and life cycle pattern. The insect goes through a complete metamorphosis and develops sequentially from egg to larva to pupa to adult. Eggs hatch within two to three weeks of being laid. Spider beetles typically complete three molts or instars during the larval stage and then spin cocoons in preparation for the pupal stage of the life cycle.

Once in the cocoon, the insect may pupate immediately or go into an extended diapause, which lasts as long as eight months. Though pupae usually reach adulthood within 20 days, the beetles remain in the cocoon for the duration of a month until they reach sexual maturity. Adults feed and mate after emerging and may live for several months. Females usually live longer than males and produce approximately two generations of offspring each year, depending on the temperature and humidity in the environment.

Problems Caused by Spider Beetles

Spider beetles forage for and contaminate human food supplies. Since the pests feed on feces and may inhabit bird and rodent nests, they are typically unclean. As such, the unsanitary insects can cause economic losses by tainting and spoiling products when they infest warehouses, mills, and grain storage facilities. Additionally, since spider beetles also feed on fabrics and paper, they pose threats to priceless artifacts in museums and in storage facilities. However, the pests do not spread diseases to humans or cause significant structural damage.

Detection / Signs of Infestation

As spider beetles are very small, generally reclusive, and active primarily at night, humans are often unaware of the presence of the pests in buildings. The most telling sign of an infestation is the sighting of an adult spider beetle or a pupal cocoon. In addition, the foraging pests may leave behind accumulations of granular materials in pantries and around food supplies.

Prevention Tips

Eradicating spider beetle infestations centres on discarding contaminated foodstuffs and eliminating access to other preferred food sources. As the pests feed on a wide variety of items and materials, preventative measures prove fairly challenging to implement. Purchasing airtight containers to store foods, vacuuming regularly, and removing rodent and bird nests from inside and around buildings are good places to start. For further assistance and control, consider contacting a pest control professional.

Control / Removal

If an infestation of spider beetles already exists in the home, the first step to removing the problem is tracking down the food source. Place sticky traps near known and suspected food sources to track the activity of the elusive pests. Any contaminated food should be promptly removed. For especially large infestations, seek the services of an experienced pest control professional.

Pictures of spider beetles