Smooth Spider Beetles Identification Guide

Smooth Spider Beetle Identification Guide

Physical Characteristics

The smooth spider beetle (Gibbium aequinoctiale) is a tiny beetle that looks more like a mite or a tick. Its long legs and round body give it a spider-like look, which is how the group got its name. Many people first spot one crawling across a bathroom floor or kitchen counter and think it is a tick.

Key features of adults:

• Size: About 1.5 to 3.5 mm long, or roughly 1/16 to 1/8 inch.
• Body shape: Round and humped, like a tiny glossy bead. The wing covers are fused into one smooth dome.
• Color: Shiny reddish-brown to dark reddish-brown. Fresh adults can look almost translucent amber.
• No hair or markings: The shell is bare and polished. The small head and thorax are also hairless.
• Legs and antennae: Long and covered in fine yellowish or golden hairs, which stand out against the dark body.
• No flight: The wing covers are fused, and the beetle cannot fly.

Larvae are small, cream-colored, C-shaped grubs with light brown heads. They reach about 3.5 to 4 mm long and stay hidden inside food or debris. Before they turn into adults, they spin a small silk cocoon.

Common Species

The smooth spider beetle is one of several spider beetles that turn up in North American homes. For an overview of the whole group, see our spider beetles guide. Related species include:

• American spider beetle (Mezium americanum): Also round and glossy, but nearly black, with golden scales on the head and thorax. See our American spider beetle page.
• Shiny spider beetle (Mezium affine): A close cousin of the American spider beetle. It is also round and glossy, but its head and thorax are covered in pale scales. See our shiny spider beetle page.
• White-marked spider beetle (Ptinus fur): Longer and hairy, with small white patches on the wing covers. See our white-marked spider beetle page.
• Hump spider beetle (Gibbium psylloides): A close relative from the Old World. Older North American records under this name almost always refer to the smooth spider beetle.

A note on names: some fact sheets call Gibbium aequinoctiale the “shiny spider beetle.” When in doubt, look at the color and the head. An all-reddish beetle with a bare head is Gibbium.

How to Tell Smooth Spider Beetles From Look-Alikes

• Ticks such as the brown dog tick: Ticks have eight legs, no antennae, and a flat body before feeding. Smooth spider beetles have six legs, long antennae, and a hard, rounded shell.
• Bed bugs: Bed bugs are flat, oval, and rusty brown, with short legs. They hide near beds and feed on blood. Spider beetles are round and domed, have long legs, and do not bite.
• American spider beetles: Nearly black, with a scaly golden front end. The smooth spider beetle is reddish all over and bare.
• Shiny spider beetles: Also glossy and round, but the head, thorax, and legs are covered in pale scales. The smooth spider beetle has no scales.
• Spiders and mites: Both have eight legs. Spider beetles have six legs and a beetle’s hard shell.

Not sure what you found? Catch one on clear tape and count the legs with a magnifying glass. Six legs, long antennae, and a smooth red dome point to this beetle.

Smooth Spider Beetle Behavior and Biology

Habitat

Smooth spider beetles live around the world in homes, stores, warehouses, and museums. In North America, most sightings come from cities in the Northeast, the Mid-Atlantic, and the Midwest. New York, Philadelphia, Chicago, Baltimore, and Washington, DC report many of them. They also turn up in Ontario and Quebec, and less often in the South and West.

They prefer dark, damp spots and avoid light. Indoors, look for them in:

• Wall voids, especially near plumbing or past leaks
• Cracks in floors and under baseboards, sinks, and tubs
• Old rodent or bird nests in walls, attics, and crawl spaces
• Pantries with old flour, grains, spices, or dried fruit
• Pet food, birdseed, and feed bags in garages and basements

Many reports come from older row homes and apartment buildings. In buildings with shared walls, the beetles may move between units through gaps around pipes.

Diet

Smooth spider beetles are scavengers. They eat almost any dry plant or animal matter. Known foods include grain products, seeds, dried fruit, dead insects, rodent and bat droppings, feathers, leather, wool, and silk. They readily feed on food that has been spoiled by moisture.

Their varied diet is why the source can be hard to find. A small pile of crumbs or dead insects inside a wall can support a steady population.

Life Cycle

The smooth spider beetle goes through four stages: egg, larva, pupa, and adult.

1. Eggs are laid in or near the food source.
2. Larvae feed hidden inside the food or debris for several weeks.
3. Pupae form inside a silk cocoon. This stage lasts about three to four weeks.
4. Adults emerge, mate, and search for food at night.

In warm conditions, the whole cycle can take as little as six to seven weeks. In cooler homes it can take several months. Two generations a year is common in temperate climates.

Hardiness and Activity

This beetle is tough. Its shell holds in water well. Adults can live in dry places for up to three months with no food or water. Adults can live up to about a year. They also handle cool temperatures better than most pantry pests and stay active below 50°F.

Smooth spider beetles are active at night. When disturbed, they often pull in their legs and play dead. In the Mid-Atlantic, sightings rise in spring and peak through summer, but in heated buildings they can show up in any month.

Treatment Methods for Smooth Spider Beetles

Smooth spider beetles do not damage the structure of your home. The goal is to find what they are eating and remove it. Spraying alone will not work while the food source is still there. These steps work well:

• Inspect stored food: Check pantry items, pet food, and birdseed for live beetles, larvae, cocoons, or fine powder at the bottom of bags.
• Look beyond the pantry: This species often breeds outside the pantry. Check under sinks, along baseboards, and around pipes, and look in basements and crawl spaces for damp debris, dead insects, and old nests.
• Clean and store food well: Vacuum shelves and cracks, and empty the vacuum outside. Store dry goods in sealed glass or hard plastic containers. Freeze questionable food at 0°F for at least four days.
• Fix moisture: Repair leaks and dry out damp areas under sinks and in basements. Seal gaps around pipes that lead into wall voids.
• Professional help: A pest control company can start with an inspection to trace the source. Targeted crack-and-crevice treatments can help once the source is gone. If mice or rats left nests or droppings behind, address the rodent problem too.

Sticky traps placed along walls and near cabinets are a simple way to track where the beetles are coming from. If you keep finding beetles, moths, or weevils in your kitchen, our pantry pest control page explains how we help with ongoing pantry problems.

References

• Spider Beetles from Penn State Extension
• Spider Beetles from the University of Florida Department of Entomology and Nematology
• Shiny Spider Beetle Fact Sheet (Gibbium aequinoctiale) from MuseumPests.net
• Spider Beetles from University of Maine Cooperative Extension
• Spider Beetle Fact Sheet from the U.S. Army Public Health Center
• High temperature effects on water loss and survival of the spider beetles Mezium affine and Gibbium aequinoctiale from the Journal of Insect Science (via PubMed Central)