Pan Traps
Introduction
Pan traps are shallow relatively large containers, compared to pitfall traps, that are placed on or sunk into the ground to sample arthropods. In their simplest form they function in nearly the same manner as pitfall traps, collecting arthropods that fall into the container and drown in water. Pan traps not only collect larger samples of arthropods than pitfall traps, they also differ with respect to the light response of the diurnal (day-active) arthropod species that are captured. Pan traps collect more specimens than pitfall traps because of the increased surface perimeter of the trap. Although both pitfall and pan traps are neutral with respect to the light response of nocturnal (night-active) species, they differ in the light response of the diurnal species captured. Covered pitfall traps collect diurnal, surface-active species with a negative response to light. Unmodified pan traps remain neutral for surface-active diurnal arthropods, collecting diurnal species with either positive or negative response to light.
The most common modification to pan traps is the colour. Insects, particularly flying insects, are differentially attracted to a variety of colours, but most are attracted to bright yellow. As a result pan traps are usually coloured bright yellow to attract a broad spectrum of low-flying insects. Pan traps are most effective if sunk into the ground flush with the top rim of the trap. Water is placed in the trap 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.
Pan traps can be used in just about any terrestrial habitat including water surfaces of wetlands, ponds or lakes (floating pan traps) or suspended in tree canopies, but the most common application is in a soil substrate for surface-active fauna. The use of pan traps in conjunction with covered pitfall traps will provide information on two different arthropod components in the same micro-habitat. Similarly, pan traps can be used in conjunction with Malaise traps to provide more information on the faunal components than would be available from traps operating independently (see Malaise traps for more information). 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 pan 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) pan 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 pan 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.
Equipment
Pan traps: plastic bowls.
Dimensions: 273 mm diameter x 79 mm depth (10.75 x 3.125 ins.).
Colour: Safety Yellow in North America (Munsell Book of Colours, Glossy Collection - 5Y 8/12).
Shovel.
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.
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
Pan traps should be sunk into the ground with their rims level with the soil or litter surface. If abundance measures are desired choose either litter or soil and use that surface consistently to standardize pan trap placement.
- Use a shovel cut through and remove the root mat and soil to create a depression which should closely approximate the size of the pan trap. 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 pan trap is placed in the depression 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.
- Fill the traps about 3/4 with water and enough salt to make a saturated solution. About 5 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 a marker containing the site and trap numbers (in water proof ink or pencil) near the trap.
Trap Servicing
Traps are serviced by scooping out the contents with an aquarium net and then either reusing the old solution or replacing it with fresh water, salt and detergent. Before scooping remove large objects such as leaves, twigs, small animals, etc. that may have fallen into the trap. The net should be dragged gently near the bottom in one direction several times until the contents are recovered. Check the trap to see if it is still level, top up with water (if necessary), and add more salt and detergent at each servicing. Ensure that the solution is transparent enough for the yellow background to show through.
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
Aguiar and Sharkov (1997), Blades and Marshall (1994), Disney et al. (1982), Finch (1991), Finnamore (1994), Greenslade (1964), Luff (1975), Marshall et al. (1994), Masner (1996), Snider and Snider (1986), Topping (1993).
|
