Spider beetles, belonging to the family Anobiidae, are small pests that can infest homes, warehouses, and other storage areas. They are not common pests, but can become numerous in certain situations. While they might resemble spiders, they are distinctly beetles with unique characteristics and habits. Understanding their diet and habitat is crucial for effective control and prevention.
Appearance and Identification
Spider beetles are generally two to five mm long, oval or cylindrical, long-legged and brown in color. Their distinctive shape, often globular or oval, helps distinguish them from other pests. They have globular abdomens and heads that remain hidden from view when observed from above. Each spider beetle has six legs and a pair of long antennae.
Several species of spider beetles exist, including:
- American Spider Beetle (Mezium americanum): The adult American spider beetle is from 1.5 to 3.5 millimeters in length and has a dark reddish-brown to black, shiny, globular abdomen. The head and legs are cream colored. Its head and thorax are covered with small hairs. The beetle’s name comes from the shape of the adult beetle, which has a "humped" appearance. When viewed from above, it resembles a spider.
- Whitemarked Spider Beetle: The adult whitemarked spider beetle is from 2 to 4.3 millimeters long and is light brown in color with patches of white hairs on the wing covers. However, the white patches are frequently rubbed off of older beetles.
- Smooth Spider Beetle: The smooth spider beetle adult is similar in shape and color to the American spider beetle. However, the head, thorax, legs, and antennae are the same color as the abdomen.
The larvae of all three species are "C" shaped and cream-colored with light brown heads.
Habitat and Distribution
Spider beetles are largely cosmopolitan in distribution and are 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. They are attracted to moist, damp areas and can become pests in mills, food processing facilities, and museums. The adult beetles are usually active at night or in dark places, so they are difficult to see.
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They can be found within walls and attics, in the cracks of wooden floors in granaries, bakeries, and such, and in the nests of birds and rodents. Spider beetles can infest bird and rodent nests. If the nest happens to be in the attic or crawl space of a home, the beetles can easily move into the living space.
The American Spider beetle resides in North and Central America and Australia. This beetle is most likely to be seen in homes and is considered one of about 30 species of beetles that are serious pests indoors for their food foraging habits. American Spider beetles may also be found in warehouses and stores. The American spider beetle is adapted to living in dry environments and cannot fly because its forewings are fused and hardened. These insects often breed in deposits of bird, pet, and rodent feces. They are also commonly found breeding in bat roosts.
Diet and Feeding Habits
Spider beetles are scavengers both in the adult and in the white, fleshy, grub-like larval stage. They are general scavengers with a big appetite, feeding on a variety of foods and organic materials, including damaged goods and organic waste. They prefer damp locations and will readily feed on foodstuffs spoiled by moisture.
Spider beetles eat mostly grains and dried stored products, but they also have been known to eat dried vegetables, spices, pet foods, fabric, nuts, animal skins, beans, rice, cereals, paper and wood, dead insects, dried fruit, feathers and hair. When food is not available, these insects will go into dormancy and will survive without food for numerous years, waking up when conditions improve and food sources are available again.
American spider beetles eat products that are moldy or products that have been stored too long to be usable. Adult spider beetles deposit their eggs in the material that the larvae will eat after they hatch. This may be broken grain or flour and grain products. They also feed on hairs, feathers, and even waste from birds or mammals like old rodent droppings and other debris depending on location.
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Life Cycle
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.
Female American Spider beetles lay about 40 eggs at a time, often inside stored foods. She spins a silken spindle-shaped cocoon out of which young beetles change into adults over the course of about three months. The American Spider beetle molts three times before its pupa stage. Larvae “hibernate” in the cocoon then emerge in the spring as fully grown beetles.
Eggs hatch within two to three weeks of being laid. 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.
Detecting an Infestation
Detecting a spider beetle infestation can be tricky, as these pests are most active at night and often go unnoticed until their numbers grow.
One of the first signs you might spot is the presence of adult beetles with long legs and rounded bodies, which can resemble tiny spiders scurrying near stored food or in damp areas of your living space. Keep an eye out for small holes in food packaging. These are often made by spider beetles or their larvae as they feed. You might also find them in bird nests or in places where there are dead insects, animal droppings, or even rodent nests, as these provide both food and shelter.
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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. Spider beetles prefer to forage at night or in dark locations and are consequently seldom seen.
Control and Prevention
Control of spider beetles relies on the discovery and elimination of the infested foodstuff. In many ways, this is similar to the control of all stored-product pests, but is made exceedingly difficult because of the beetles' ability to feed on so many different items. 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.
Follow these strategies to help with American beetle control and prevention:
- Find out where the beetles are coming from: It is important to locate any bird, pet, rodent, or bat feces that may be the source of infestation. These must quickly be eliminated.
- Clean up: Throw away all infested food. Discard any trash, food products, even decorations that have been infested.
- Tidy Up!: Remove all clutter from your home. Clean any cabinets and shelves where contaminated food products were stored. Vacuum all infested areas then wash them with soap and water.
- Vacuum: Vacuum pantry and cabinet shelves thoroughly. This will remove any food residue that larvae might eat. Vacuuming will also remove any beetles that are still in their cocoons.
- Practice good food storage habits: Store new food products in sealed containers. Keep dried goods like cereals, rice, birdseed, and pasta in containers with close-fitting lids. As a precaution against reinfestation, you may want to store susceptible foods in sealable glass, metal or heavy plastic containers or in the refrigerator or freezer until you are convinced the infestation is gone.
- Locate the food source: If an infestation of spider beetles already exists in the home, the first step to removing the problem is tracking down the food source. If the food source for the infestation cannot be found, place monitors (sticky traps) in the vicinity where beetles were seen. When beetles are captured on a trap(s) in one area, move more traps to that section. Continue to focus on the movement of the traps until the infestation is located. All efforts should be made to remove the infested material.
Professional Assistance
With this particular species, there is usually an underlying cause of the beetle infestation, which can be the most difficult part of extermination. A professional will locate nesting sites and food sources of the American spider beetle first, then set to work eliminating both.
Controlling spider beetles in homes starts with a thorough inspection. Insecticide will be most effective if it is applied into cracks and crevices or voids. A light spray of household insecticide can be applied to cracks and corners of infested storage areas and may help eliminate any beetles that remain after the thorough cleaning. For product selection and treatment, it is usually best to contact your pest control professional.
Whether or not you employ professional services should be dependent on the scope of your American spider beetle problem. Keep in mind that the root of the problem may be another pest like rodents, birds, or bats and they should be eliminated first.
Impact and Significance
Spider beetles contaminate human food supplies and typically unclean, which can make food inedible in homes, and have serious economic consequences for businesses. Not only does this species damage food products and create an unsanitary environment for food, it can also inflict structural damage as it infests wall voids and ceilings. American Spider beetles are not known to bite humans, and rarely come in close contact with them, but will readily enter areas where food is stored.