A pitfall trap is one of the simplest and most effective ways to catch ground-dwelling insects and other invertebrates. You need a container buried flush with the soil surface so that crawling creatures fall in as they walk across the ground. The basic version takes about 10 minutes to set up with materials you probably already have, and more advanced designs can significantly increase what you catch.
Basic Materials and Setup
At its simplest, a pitfall trap is a smooth-sided cup or container sunk into the ground so the rim sits perfectly level with the surrounding soil. Plastic drinking cups, yogurt containers, or mason jars all work. The key is that the rim doesn’t stick up above the surface, because even a small lip will deflect insects around it rather than into it.
To install one, dig a hole slightly wider than your container, press the container in, and pack soil tightly around the outside so there are no gaps. Gaps create a ledge that insects will sense and avoid. The soil surface should flow smoothly right up to the container’s edge. If you disturb the surrounding area during digging, tamp it down and scatter some loose leaf litter around the trap to restore a natural appearance.
Choosing the Right Size
Trap diameter has a direct effect on what you’ll catch. Research comparing traps of 3 cm, 5 cm, and 12 cm diameter found that the smallest traps were significantly less effective, capturing fewer species and biasing the catch toward tiny-bodied animals. Long-legged creatures like harvestmen simply stride over a small opening, and most millipede species (with body lengths over 20 mm) can evade or escape a 3 cm trap entirely.
Medium traps (around 5 cm diameter) and large traps (around 12 cm) performed similarly well for both species richness and total captures. A widely cited recommendation for standardized biodiversity monitoring suggests a diameter of 9 to 10 cm, which is roughly the size of a standard plastic drinking cup. That’s a good target for general-purpose trapping. If you’re specifically after beetles, spiders, or other small arthropods in a garden, a standard 7 to 9 cm cup works well. Go larger if you want to capture bigger ground-dwellers like millipedes or large ground beetles.
What to Put in the Trap
What goes in the bottom of your container depends on whether you want to keep specimens alive or preserve them.
For live capture: Leave the trap dry or add a thin layer (2 to 3 cm) of loose soil, sand, or leaf litter in the bottom. This gives captured animals something to hide under, which reduces stress and protects them from ants and temperature extremes. Small pieces of bark or a short section of PVC pipe provide additional shelter. In dry conditions, a few pieces of damp sponge help prevent dehydration. Place a small floating platform (a piece of wood or polystyrene) in the trap if rain is possible, so animals won’t drown in pooled water.
For collecting preserved specimens: Fill the bottom of the trap with about 2 to 3 cm of preserving liquid. For a simple home or school project, water with a few drops of dish soap works. The soap breaks the surface tension so insects sink rather than walking on the water’s surface. For more serious sampling, a 50% solution of propylene glycol (a nontoxic antifreeze available at hardware stores) is the most widely recommended preservative. It kills specimens quickly, prevents decomposition, and doesn’t evaporate as fast as water. Research comparing six different preservatives found that propylene glycol, ethanol, and acetic acid all captured significantly more beetles than plain water or saline.
Adding a Rain Cover
Rain will flood an uncovered trap within hours, washing out specimens or drowning live captures. A simple and effective rain cover is a square of plywood (about 20 cm across) supported a few centimeters above the trap by four large nails pushed into the ground at the corners. This keeps rain and falling debris out while leaving enough clearance for ground-crawling insects to walk underneath and fall in. A flat rock propped up on smaller stones works in a pinch.
In especially rainy conditions, mound the soil slightly around the trap so the rim sits just a hair above the surrounding grade. This redirects surface water flow away from the opening without creating a noticeable barrier to crawling insects.
Using Drift Fences to Catch More
A drift fence is a short, low barrier that guides wandering insects toward your trap. Animals walking along the ground bump into the fence, follow it, and eventually fall into a trap placed at one or both ends. Drift fences are typically 5 to 15 meters long and made from thin plywood, aluminum flashing, or even stiff cardboard pushed a few centimeters into the soil so insects can’t crawl under it. The fence only needs to be about 10 to 15 cm tall for invertebrates.
Place a pitfall trap flush with the ground at each end of the fence. This setup dramatically increases capture rates compared to standalone traps, especially for fast-moving ground beetles and spiders that would otherwise walk right past an isolated cup.
Using Bait
Unbaited traps catch whatever happens to wander by, but adding bait lets you target specific groups. A small piece of overripe fruit or rotting banana attracts fruit-feeding beetles and flies. A bit of raw meat or fish attracts carrion beetles. For dung beetles, animal dung placed in or near the trap is the standard approach. Omnivore dung generally attracts a wider range of species than herbivore dung. Research in tropical forests found that pig dung performed just as well as human dung for sampling dung beetle diversity, and most dung beetle species turned out to be generalists attracted to many bait types.
Suspend bait above the trap rather than dropping it directly into the preserving fluid. A small mesh bag or perforated container hung from the rain cover or a stick works well. This keeps the bait from contaminating your collection while still drawing insects to the area.
When and How Often to Check
Many ground-dwelling invertebrates are most active at night, so setting your trap in the evening and checking it the following morning is ideal. Don’t leave a trap unchecked for longer than overnight if you’re doing live capture. Heat, dehydration, predation by ants, and exposure can kill trapped animals quickly once daytime temperatures rise.
If you set a trap during the day, check it at least every few hours, and ideally every hour in warm weather. Traps with preservative fluid can be left longer (researchers sometimes run them for a week or more between checks), but you’ll get cleaner specimens and fewer scavenger problems with more frequent visits.
Reducing Harm to Non-Target Animals
Pitfall traps can accidentally capture small mammals, frogs, lizards, and other animals you aren’t targeting. A few design choices minimize this risk. Keeping your trap diameter at 9 to 10 cm or smaller makes it physically impossible for most vertebrates to fall in. If you’re using larger traps, place a stick or rough-surfaced ramp (a strip of bark works well) leaning from the bottom of the trap to the rim. Insects are too small to climb out, but mice, frogs, and lizards can use the ramp to escape.
Covering traps when not in active use is important. Simply place a flat object over the opening or fill the hole temporarily. An unattended open trap left for days can become a death trap for small animals. If you’re trapping in an area with shrews, some researchers cut a small “shrew hole” in the side of the container near the top, sized to let tiny mammals squeeze out but too high for ground beetles to reach.
Legal Considerations
Small-scale pitfall traps for catching insects in your garden or for school projects are generally unregulated. However, larger pitfall traps designed to capture vertebrates (mammals, reptiles, amphibians) fall under wildlife trapping laws in most places, and those laws vary dramatically. Pit traps for animals are outright banned in several U.S. states, including Alaska, where the prohibition specifically targets pit-style traps for canines and other furbearers. Arizona prohibits most trap types on all public land. Michigan lists pitfalls among illegal methods for taking game.
Even in states where some primitive trapping is permitted, you’ll typically need a trapping license, and there may be restrictions on trap size, location, and which species you can target. If your project involves anything beyond catching garden insects, check with your local wildlife agency before setting traps. The regulations that matter most are the ones your local game warden enforces, and interpretations can vary by region.