Pitfall traps
Pitfall traps are commonly used for sampling the many types of invertebrates that move over the ground surface, such as beetles, ants, spiders, woodlice and millipedes. There are many designs of pitfall traps, but all work on the principle of a slippery-walled container sunk into the ground so the lip is flush with the ground surface. As a general rule, larger diameter traps appear better for sampling larger sized invertebrates, but in difficult ground conditions are more difficult to install than smaller traps.
Some designs have an internal cup that fits within an external pipe or vessel so the trap can be emptied and reset quickly without having to dig up the entire unit. Pitfall traps normally have a preservative fluid into which the animals fall (e.g., 30% monopropylene glycol), and this prevents larger predators like spiders from destroying the smaller specimens in the trap. We also recommend pitfall traps have some form of cover suspended about 1 cm over the trap to help keep out rain, leaves and other debris that could enable specimens to escape, but still allows the movement of small animals underneath. If minor surface flooding is likely to be a problem, the addition of small drain holes about 2 cm below the lip of the trap can allow excess water to escape without having the sample flow out the top.
For surveillance of ants and other small invertebrates, we have found an excellent pitfall trap can be made from cheap plastic vials of about 25 mm diameter and 55 mm depth. A small soil corer of the same diameter as the vial allows us to deploy a large number of pitfall traps rapidly and snugly without having to spend much time neatening up the edges of each trap, as is the case with larger hand-dug designs. A slightly more expensive option is to dip the top half of the pottles in an anti-friction coating such as teflon in order to minimise the risk of animals escaping. We have found such teflon-coated traps regularly catch quite large beetles and spiders (sometimes longer in body length than the diameter of the trap), as well as being very effective for sampling ant communities. A plastic jam jar lid of about 65 mm diameter makes an excellent cover if large sections of the side walls are cut out to enable invertebrates to move beneath. These lids can be pegged down with wire or held down by the weight of a small rock
When it is time to service the traps, the vial can be pulled out of the ground, a label inserted, and the screw cap fitted. The sample can then be sorted or transferred into alcohol back at the lab. A new pitfall can then be quickly deployed close by with the aid of the corer, and the dirt plug from the new hole used to fill up the old.