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Pitfall Traps

An effective method for invertebrate capture

By Dave King

Introduction:

One of the most effective ways of collecting invertebrates during fauna surveys is by the use of pit-fall traps. The trap described here-in was designed by the writer as an inexpensive and easily made item, at the same time very effective in overcoming a number of inherent problems.

pitfall

Problems associated with pit-fall traps intended for invertebrates are:-

  1. Catching non target species, such as frogs.
  2. Rain water getting in and diluting the preservative.
  3. Litter in the form of leaves, twigs, etc. dropping or blowing in.
  4. Evaporation of the preservative.

To overcome the above problems the trap has a wire mesh screen around the pit-fall container, a cover to prevent rain water and debris falling in. The preservative used is ordinary motor vehicle anti-freeze, being non-evaporative, reasonably cheap and readily available. This anti-freeze fluid is of the order 95% pure ethylene glycol plus additives of corrosion inhibitors, colour dyes etc., none of which, in the writers experience, adversely effect the collected material.

Materials & Construction:

The lid or cover consists of the lid from a standard plastic margarine container. Wire mesh, of 1cm mesh, readily obtainable from most hardware stores, is cut into strips about 30mm wide and length sufficient to form a cylinder that will fit into the groove around the lid perimeter. For the average margarine container this will be about 360mm. When cutting this strip, the top edge is cut to allow a wire strand as the rim, the opposite edge is cut allowing the ends of the vertical wires to be free, shown in the diagram. The lid is secured to the mesh cylinder by passing a length of wire through the rim of the lid , bending each end to retain it. Make certain the securing wire passes under the top wire of the mesh. Holes for this wire can be made using a heated wire.

A normal plastic disposable coffee cup is used for the container, buried flush with the soil surface. A second cup can be used as a permanent and stable hole in which to place the trapping cup. This makes it much easier to replace the cup after emptying and recharging.

Operation:
Placement of these traps is probably best in situations where ground vegetation is minimal or relatively bare. Camouflage with other vegetation is generally advisable to reduce adverse aesthetic values and inquisitive public. Traps can be left unattended for a week or more without deleterious effects upon any specimens captured.

Recovery of specimens is performed by straining the contents through a kitchen plastic sieve with fine mesh. The resultant collection is then carefully washed in water, to remove the glycol, then transferred to a shallow dish containing a 70% solution of alcohol for sorting of the specimens. Finally, transfer to a fresh 70% alcohol solution before further processing.