American Spider Beetle

Mezium Americanum

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Identification and Life Cycle

Spider beetles are small beetles belonging to the Anobiidae family. The more common species is the American spider beetle (mezium americanum), which resembles a small spider due to its long legs and large, pear-shaped rounded abdomen. They are also sometimes confused for bed bugs.

The American spider beetle or black spider beetle is approximately 1/16 to 3/16 – inch in length, dark reddish-brown to black in color with six legs. The head, thorax, legs, and antennae (looks like a fourth pair of legs) are covered with tiny scale-like pale yellow hairs. Adult spider beetles, when disturbed, often will draw their legs tightly toward their bodies giving the appearance they are dead.

The female adult spider beetle can lay approximately 120 eggs over 3-4 weeks in the early summer months. She will lay eggs in the same substance that the larvae will eat after they hatch. The eggs may be laid on the outside of grain sacks or on dry food debris hidden in cracks and crevices. The larvae are about 1/8-inch in length, C-shaped, whitish to cream in color with short legs and a brown head. The life cycle from an egg to adult takes approximately 3-7 months. An individual spider beetle can live up to 12 months.

Habits and Damage

Spider beetles are more commonly found in homes and warehouses. They feed on a variety of stored products, and can live in attics, behind walls, in drop ceilings and between wooden boards. They prefer to live in cool, damp locations with high humidity.

They are non-flying nocturnal insects that mostly feed at night on food products that are spoiled by moisture. They can be found in pantries on stored foods such as almonds, beans, cereals, flour, wheat, dried fruits, corn meal, and on animal-based materials including textile fabrics, leather, feathers, silk and wool. These scavengers will also feed on dead insects and rodent droppings, and can infest abandoned bird and rodent nests.

The adult spider beetle may gnaw into cardboard packages storing dried products, but prefer unpackaged plant or animal material. The larvae can bore small holes into cardboard packages and wood to pupate. In doing so, stored food products may become covered by the silken pupa cocoons spun by the larvae. They can contaminate stored products with their feces, pupa cases, and body parts.

The spider beetle does not bite or carry any known diseases, although to allergically sensitive people, it may cause a skin reaction such as a welt resembling a “bite mark”.

Common Signs Of Spider Beetle Activity

  1. Holes in boxes or food packages. Spider beetles can bore 1 mm to 2 mm size holes in cardboard boxes.
  2. Webbing on food.  The larvae spin and leave behind silk-like webbing on the surface of foods as they eat.
  3. Silken Cocoons. Spider beetles spin silken cocoons where its larvae will change into adult beetles.
  4. Eggs & Eggshells. Can be about the size of a head of a pin and are a translucent pearl-white color.

Prevention and Management

The best strategy for spider control is sanitation and inspection. An Integrated Pest Management (IPM) program can properly identify the pest and help you maintain a spider beetle-free environment.

It is important to recognize and alter conditions in and around a home or building to limit areas where spider beetles can gain access from outside.

Exclusion. Limit areas where spider beetles can gain access into buildings, especially around doors & windows.

Sanitation. Reduce the possibility of an infestation by eliminating sources of moisture and food.

The following steps can be taken to help prevent or reduce a potential problem with spider beetles.

    • Provide suitable drainage for basements, cellars, and crawl spaces under the building to reduce moisture.
    • Dry out damp areas by using a dehumidifier.
    • Fill all visible cracks and voids in the foundation as well as using a sealant or weather-stripping around windows, doors and screens.
    • Install door sweepers to insure that doors close tightly.
    • Keep food storage areas clean. Do not allow crumbs or spilled food to accumulate.
    • Keep kitchen cabinets & shelves clean of any spilled food products.
    • Store dry goods such as rice, cereal, pasta, birdseed, pet food etc. in tightly sealed containers.
    • Locate & eliminate immediately any bird, rodent or bat feces that may be the source of the infestation. Remove any empty bird nests.

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