Some features influencing the efficiency of pitfall traps

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Abstract
Some of the trap features influencing the ability of pitfall traps to catch Coleoptera, especially Carabidae, were investigated by field and laboratory experiments. It is shown theoretically that the catch of circular and rectangular traps can be compared on a basis of their perimeter lengths. Formulae are given for the reduction in catch per trap when traps are placed close together in a grid. Field experiments compared the catch of six types of trap, from 2.5 cm diameter glass tubes, to 2 m long sections of guttering. Glass traps were most efficient in proportion to their size; plastic and metal traps had a reduced catch. Small traps were most efficient in catching small species, whereas large traps caught relatively more large beetles. The capture efficiency of each type of trap was tested in the laboratory, using six species of Carabidae. Traps caught about 75% of beetles that contacted their perimeter; only small traps caught small beetles efficiently; the largest species was caught poorly by nearly all traps. The rate of escape, from each type of trap, of small, medium and large sized Carabidae was tested. Escape was negligible from glass-sided traps, but was about 4% per day from plastic traps, and 10% from metal ones. Large beetles escaped most from small traps, and small beetles from large ones. The observed differences, between types of trap, in capture and retaining efficiencies accounted for some, but not all, of the variations in catch found in the field.