Pitfall Traps
Introduction
Pitfall traps are deep relatively small containers (cups), compared to pan traps, that are sunk into the ground to sample surface-active arthropods. They function in a manner similar to pan traps by collecting arthropods that fall into the container and drown in water at the bottom of the trap. Pitfall traps collect smaller samples of arthropods than pan traps because of the smaller top perimeter of the pitfall trap. They are easier to install and service because of the smaller volume of water needed to operate the trap, less expensive in materials to construct the trap, and less expensive in labour to process the smaller samples. Covered pitfall traps not only differ from pan traps in the size of the sample collected but also in the light response of the diurnal (day-active) species collected. Covered pitfall traps tend to collect diurnal arthropods with a negative response to light while pan traps tend to be neutral for diurnal surface-active arthropods.Pitfall traps are deep relatively small containers (cups), compared to pan traps, that are sunk into the ground to sample surface-active arthropods. They function in a manner similar to pan traps by collecting arthropods that fall into the container and drown in water at the bottom of the trap. Pitfall traps collect smaller samples of arthropods than pan traps because of the smaller top perimeter of the pitfall trap. They are easier to install and service because of the smaller volume of water needed to operate the trap, less expensive in materials to construct the trap, and less expensive in labour to process the smaller samples. Covered pitfall traps not only differ from pan traps in the size of the sample collected but also in the light response of the diurnal (day-active) species collected. Covered pitfall traps tend to collect diurnal arthropods with a negative response to light while pan traps tend to be neutral for diurnal surface-active arthropods.
The most common modification to pitfall traps is a cover. The cover creates a cave- like microhabitat that is attractive to surface-active diurnal arthropods with a negative response to light. The cover also protects the trap and its contents from climatic conditions like heat and precipitation, thereby providing some buffering capacity from decomposition for the sample. The cover should be opaque to avoid a greenhouse effect in the trap, and light grey in colour so as not to attract low-flying insects or to attract a different suite of species than that captured by yellow pan traps. The cover is made of ceramic tile, a material heavy enough not to be dislodged by wind, or affected by rain or sun, and unpalatable to termites. The cover is supported 2.5 cm (1 inch) above the pitfall trap by 4 galvanized nails or plastic rods. The supporting nails or rods should be environmentally neutral so as to minimize any effect on the soil chemistry and biota. Pitfall traps must be sunk into the ground flush with the top rim of the plastic cup. Water is placed in the trap and acts as a collecting medium, and liquid detergent is added as a surfactant to break the surface tension of the water causing the insects to drown. Salt is added as a preservative.
Pitfall traps can be used in any ground level substrate except water and rock. They can be used to sample arthropods in animal burrows, litter, rotting wood, and peatlands, but the most common application is in a soil substrate for surface-active arthropod fauna. The use of pitfall traps in conjunction with other sampling protocols, pan traps, Malaise traps, or Berlese extractors, will provide information on different arthropod components in the same micro-habitat. In order to fully sample any site or habitat, trapping should be continued through the whole of the active season, usually April to October in temperate latitudes. Such an extended trapping period is needed to accommodate the different phenologies exhibited by the various taxa.
In species richness studies the number of species obtained by pitfall traps will be increased if traps are placed in as many microhabitats as possible. Microhabitats can be identified based on, among other things, soil particle size, amount and type of litter, surficial moisture, vegetation structure, dominant plant species, degree of shade, or a combination of these and other factors. In studies that focus on abundance measures (relative trapability) pitfall trap replicates within a microhabitat will be necessary. The number of replicates can be based on species accumulation curves and selected to achieve 85% or more of the target species. In other words, how many pitfall traps are needed to collect 85% or more of the trapable species in a given microhabitat? The number of replicates necessary to obtain 85% of species will vary depending on the taxa but for many groups in north temperate ecosystems it is between 8 and 10 traps. Statistical analyses usually require that arthropods at a site have an equal chance of entering each trap. To accommodate this requirement, traps should be placed fairly close together. The 8 to 10 individual traps set up in a circle of 10 m radius to form a "trap circle" meet these requirements.
Equipment
Pitfall traps: 450 ml plastic cups (2 per trap). Cups must fit snugly inside each other with rims touching.
Dimensions: 92 mm top diameter x 100 mm depth (3 5/8 x 4 inches).
Cover: 152 x 152 mm (6 x 6 inches) ceramic tiles (1 per trap).
Tile colour: light gray (Munsell Book of Colours, Glossy Collection - N7.5 on the Neutra l Value Scale, 50.7% reflectance).
Cover supports: 152 mm (6 inch) galvanized nails or plastic rods (eg., chopsticks, skewers) (4 per trap).
Trowel.
Water: tap water or water from a natural source filtered to remove arthropods (use aquarium net to filter water).
Water bucket or container to carry water.
Pure detergent: unscented liquid dishwashing detergent.
Salt.
Plastic or wooden markers.
Aquarium net, fine mesh.
Sample bags with twist ties. Self-sealing plastic bags (i.e. whirlpak type) are most convenient for field specimen storage.
Plastic cup (same size as used for trap).
Squeeze bottle, 0.5 l capacity.
Alcohol, 70-90% denatured ethyl or 70-90% isopropyl (rubbing alcohol).
Soft-lead pencil.
Card stock for labels.
Methods
Installation
Pitfall traps should be sunk into the ground with the top rim of the plastic cup flush with the soil or litter surface. If abundance measures are desired choose either litter or soil and use that surface consistently to standardize pitfall trap placement.
- Use trowel to cut through and remove the root mat and soil to create a hole which should closely approximate the size of the plastic cup. Care should be exercised in removing soil and root mat so as to minimize disturbance in the adjacent microhabitat.
- The excavated root mat and soil must be removed about 10 m from the site to minimize disturbance but some soil should be retained for fill around the outside of the trap.
- The pitfall trap is placed in the hole and the outside surrounding edge filled with soil from the excavation and carefully worked flush with the rim. The fill should provide a surface for microarthropods to walk unimpeded up to the rim of the trap.
- Fine soil which falls into the trap during placement can be blown out, larger
particles must be removed by hand.
- Place a second cup inside the first. It is important to select cups that fit snugly inside each other with rims touching since a second plastic cup is placed inside the first so that the outside cup will not be disturbed during servicing. Note that cups from different manufacturers are often of different dimensions. The outer container can then be left in the ground permanently, and the inner container removed for servicing.
- Insert 4 cover supports (galvanized nails or plastic rods) into the soil surrounding the trap leaving 2.5 cm (1 inch) of each support visible above the substrate to support the cover.
- Fill the traps about 3/4 with water and enough salt to make a saturated solution. A few drops of detergent are added as a surfactant. Salt is always needed in tropical climates unless the servicing frequency is 24 hours or less. In temperate climates salt is not needed if traps are emptied every 2 days or more frequently, but with longer servicing intervals (to a maximum of 7 days), salt is essential.
- Place the cover on the supports.
- Place a marker containing the site and trap numbers (in water proof ink or pencil) near the trap.
Trap Servicing
Service traps by removing the tile cover and lifting the inner cup, containing the sample, from the outer cup. Remove large objects such as leaves, twigs, small animals, etc. that may have fallen into the trap. Contents of the cup are gently swirled to lift specimens off the bottom then poured through the aquarium net, the liquid collected in another cup to avoid disturbance of the microhabitat. The liquid is then either reused or replaced with fresh water, salt and detergent. If reused, the liquid and the servicing cup can be placed back in the outer container of the trap. If not reused, the liquid should be removed and disposed about 10 m from the trap to avoid microhabitat disturbance. Replace the cover on the trap.
Specimen Processing
Specimens are transferred to plastic bags from the net using a squeeze bottle of water to remove the last specimens. Each trap should be processed separately and a label containing the site and trap identifiers, and date of collection placed in the bag. Specimens should be processed the same day in a lab or base camp by rinsing the contents of each bag in the net under a gentle stream of water for several minutes to remove dirt, salt and detergent. Pick out any remaining large debris. Fresh water washing is essential because the detergent and dirt will form a film on the insects if they are placed directly in alcohol. This film is difficult or impossible to remove once specimens are in alcohol, thus rendering the specimens much less useful. Invert the net contents into the specimen bag and using a squeeze bottle filled with alcohol gently wash contents out of net and down sides of specimen bag to the bottom. Cover the sample in the specimen bag with alcohol to at least level of sample volume. Sample labels must be placed inside the bag with the specimens. Change the alcohol in about 7 days, and if possible, refrigerate or freeze the samples to await further processing. Samples in storage should be double bagged.
Further Reading
Anderson (1982), Baars (1979), Briggs (1961), Clark (1992), Dennison and Hodkinson (1984), Dunn (1989), Gist and Crossley (1973), P. Greenslade (1973), Greenslade and Greenslade (1971), P.J.M. Greenslade (1964), Halsall and Wratten (1988), Luff (1975), Marshall et al. (1994), Mitchell (1963), Newton and Peck (1975), Niemelä et al. (1990), Rivard (1962), Scudder (1996b), Southwood (1978), Spence and Niemelä (1994).
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