Whitney Cranshaw, Colorado State University, Bugwood.org
Mezium americanum (American spider beetle), Ptinus fur (Whitemarked spider beetle), Gibbium aequinoctiale (Smooth spider beetle)
Spider beetles are small beetles belonging to the family Anobiidae. They are called spider beetles because they look somewhat like small spiders (or large mites) due to their long legs and relatively large, rounded abdomens.
Ptinids are not common pests but can become numerous in certain situations. They are general scavengers and can be found in pantries, museums, grain mills, warehouses, and attics that contain bird, rodent, or bat droppings.
The American spider beetle adult is from 1.5 to 3.5 millimeters in length and has a dark reddish-brown to black, shiny, globular abdomen. The head, thorax, legs, and antennae are covered with hairs that are a pale yellow to cream color.
The whitemarked spider beetle adult is from 2 to 4.3 millimeters long and are light brown in color with patches of white hairs on the wing covers. However, the white patches are frequently rubbed off of older beetles. Males have a thinner and more elongated abdomen than the females.
The smooth spider beetle adult (Fig. 1) 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. This species, more than the other spider beetles, resembles a large, reddish mite.
The larvae of all three species are "C" shaped and cream colored with light brown heads. The length is dependent on the age of the larvae but can reach 3.5 to 4 millimeters.
Figure 1. Adult smooth spider beetle
The three species discussed have similar life histories and have been reported to feed and reproduce on a wide variety of foodstuffs, including almonds, animal skins, beans, books, bones, brushes, cacao, cereals, chocolate powder, corn meal, dates, dead insects and insect collections, dried fruits, dried mushrooms, dried soup, drugs derived from powdered leaves, excrement (dried), feathers, figs, fish meal, flour, ginger, grains, hair, herbarium specimens, hops, leather, maize, nutmeg, old wood in houses, paprika, rye bread, rye, seeds, silk, stuffed birds, textile fabrics, various spices, wheat, and wool.
Spider beetles prefer to forage at night or in dark locations and are consequently seldom seen. They also prefer damp locations and will readily feed on foodstuffs spoiled by moisture. 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. They can be active at temperatures below freezing and will have from one to two generations per year depending on the ambient temperature and relative humidity.
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. For instance, although spider beetles may be found in the pantry, the real infestation could be in a rodent nest under the floor, in bat droppings in the attic space, or from dead overwintering insects trapped within an exterior wall.
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 the movement of the traps until the infestation is located. All effort should be made to remove the infested material. If you are unable to gain control, enlisting the aid of a pest management professional may be necessary.
Pesticides are poisonous. Read and follow directions and safety precautions on labels. Handle carefully and store in original labeled containers out of the reach of children, pets, and livestock. Dispose of empty containers right away, in a safe manner and place. Do not contaminate forage, streams, or ponds.
Authored by: Steve Jacobs, Sr. Extension Associate
October 2006 Revised December 2013