Pitfall traps

Pitfall traps can be a useful technique for confirming the presence of invasive ants in an area and assessing the impact of the ants in the environment.

The most common type of pitfall trap are those dug into the ground. But pitfall traps can also be placed in trees to capture ants.

Standard pitfall traps in the ground

Pitfall trapping involves digging a container into the ground to catch ants. The container is half filled with a liquid that drowns any insect that falls into it (and preferably preserves it also).

The ants and other insects are then counted and identified.

You can compare the results of uninvaded and invaded areas to get an idea of the impacts of the ants in the environment. Invaded areas usually have fewer (or different) ants or other insects.

Note that pitfall traps are known to trap small animals such as lizards. If you are working in a sensitive area, where by-catch is likely you should consider installing a lid (an elevated plastic disk or plastic lid larger than the trap for example) on sticks over the pitfall trap.

Choosing sites

First you need to plan where you will put your containers. Usually this means choosing a few areas (sites). Within these sites you choose plots (smaller areas within the sites) or transects (lines through the site).

Plots or transects can be any size that are convenient. All plots within a site should be the same size (for example 5 m x 5 m). Similarly, if you use transects these should all be the same length (for example 20 m).

Choose sites that are typical of the invaded area. Preferably choose site that are not too difficult to dig at. This can be a little difficult in areas with coral rubble.

Place the transects / plots at least 10 m away from the sea / lagoon. On small atoll islets this should preferably be about halfway between the ocean and the lagoon.

Replication is needed at all levels in your sampling design. So you need more than one pitfall trap in each transect or plot. How many you choose will depend on practicalities like how many containers you have and the total area you have to cover. Also remember that the more containers you have the more you need to sort out later.

You need to know that your results are sound. This is why you have many sites, many transects / plots within sites and many traps within transects / plots. This is called nested replication, and removes the effect of variation at different spatial scale (between the sites, between the transects and between the traps). 

At least two replicates at each level (site, transect / plot, trap) are recommended. The sites, transects and traps should be given names or numbers to identify them.

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For pitfall trapping you will need:
  • enough containers for all your sites. You can use a small plastic cup, cut down plastic bottle, specimen jar or pottle. Small jars with lids are best as when you remove the trap, you can just screw the lid on. Using cut down bottles or plastic cups means you have to move the insects that have been caught into another container (unless you count them as you take the traps out). The cups used need to be plastic (to avoid them breaking) and about 8 cm in diameter (or whatever is available)
  • a killing agent / preservative. Ethanol (an alcohol) is the most effective killing agent / preservative. But in tropical areas it evaporates too quickly. Propylene glycol doesn't evaporate and is not flammable, non-toxic, and therefore a safer option. You can use water with detergent (soap, dishwasher liquid) for traps that will be in the ground for shorter periods (e.g. 24 hours). Using water alone doesn't work very well as the surface tension of the water allows the caught insects to escape (detergent and liquids like propylene glycol break this surface tension) If you use water it is important to sort the traps as soon as they are recovered or the insects you have caught will start to rot. You will need enough preservative for all your sites. To figure this out multiply the number of traps by half the volume of each trap. Divide this by 1000. This is the number of litres you need. For example for 70 ml specimen jars for 100 traps: 70/2=35, 35*100=3,500, 3,500/1,000=3.5 litres
  • a trowel, spike or strong stick for digging the holes.
  • something to mark where the traps have been placed
  • small sticks with flagging tape (or bright coloured fabric) for the traps, and tape or bright fabric or paint to mark the transects / plots and sites
  • sometimes a bucket of sand from nearby (if the soil is very stony, hard to dig or coral rubble)
  • small pieces of paper (waterproof is best) for labelling the inside of traps
  • pencil for labelling the paper
  • permanent marker for labelling outside of traps
Pitfall traps can be made from cut down water bottles, plastic cups or specimen jars / pottles (© Monica Gruber, Pacific Biosecurity)
Equipment for pitfall trapping: a preservative agent, traps, a trowel, marker pen, pencil and paper, a stick with flagging tape to mark traps, and flagging tape to mark sites / transects (© Monica Gruber, Pacific Biosecurity)
Pitfall traps can be dug in with a trowel, a sharp tool or even a stick (© Warren Butcher, Pacific Biosecurity)
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Placing the traps

If it rains heavily the traps will fill with water, so it pays to put traps out when rain is unlikely.

Pitfall traps are usually spaced about 5–10 m apart. Use the same number of traps on each transect / plot and space them evenly.

  1. To make sure that soil doesn't fall into the cup as you set it:
    • if using pottles or jars with lids, turn the lid upside down on top of the jar. Leave the lid on until the jar is fully sunk into the ground. Then take the lid off gently. Using jars is much easier than using cups
    • if you are using plastic cup traps, put two cups in the hole you have dug. You then take the inner cup out
  2. Dig a hole just large enough for the container. On coral atolls this can be a little difficult
  3. The open lip must be level with the ground (or just below the surface of the ground). As ants (and other insects) run along the ground they fall into the cup and cannot get out
  4. If the ground is very stony, add extra soil or  sand to fill the gaps. Otherwise the ants might just walk between the stones and miss the trap
  5. Remove the lid
  6. Add the preservative
  7. Leave the traps for 24 - 48 hours. 24 hours is usually long enough if the invasive ants are at high abundance, and it lessens the risk of rain ruining the traps. If rain is unavoidable, you can make a lid for the trap using a plastic plate, plastic lid, or similar
  8. Collect the traps
  9. Label each trap in pencil on (preferably) waterproof paper. Include the site, transect / plot and trap (e.g. site Atafu, transect B, trap 2 would be labelled AtafuB2). Also add the date to the label. Place the label inside the trap and screw on the lid well. Also write the label on the lid of the trap in permanent marker. Labelling the outside makes it easy to check that all traps have been collected. It is important to label the inside as well in case the label on the outside rubs off

The traps should be taken for sorting and identification as soon as possible.

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1. place the lid upside down on the jar

3. make sure the soil is level with (or slightly above) the top of the jar

2. dig a hole just large enough for the trap

4. remove the lid and add the preservative (© Monica Gruber, Pacific Biosecurity)
Cross section of a pitfall trap in the ground (© Allan Burne, Pacific Biosecurity)

Sorting and identification

The traps are sorted by tipping the contents (bit by bit if there are a lot of ants / liquid) into a dish (usually a petri dish). The ants are inspected (under a microscope usually), identified if possible, and counted.

The ants ideally should be identified to species. However, this can be very difficult for the non-expert. It needs to be remembered that getting the identifications 100% exact for all the ants and insects found is NOT necessary when simply assessing impacts or abundance of the invasive ant.

The key is to be able recognise which ant is the invasive ant. The other ants can be grouped into what is known as 'morpho-species': similar looking groups.

Recording results

Recording the species identified in a table such as the one below helps in calculating the totals for all sites.

In this example, yellow crazy ants are the target species. The other ant species are only numbreed as they were simply grouped into morpho-sepcies (i.e. small brown ant, small black ant, tiny brown ant, large orange ant). All other insects were recorded as a single group.

Note that usually more than two traps would be included in a transect, but this is just an example.

Traps set: 23/11/2015 4pm-6pm Traps collected: 24/11/15 4pm-5pm      
Site Transect Trap # yellow crazy ants species 2 species 3 species 4 species 5 insect
Atafu A 1 820 1        
Atafu A 2 808           
Atafu B 1 120  2 1      
Atafu B 2 380           
Atafu 1 60   3       1
Atafu 2 20     1     4
Fogalaki 1 3 3 2     1
Fogalaki 2 12  1   1   1
Fogalaki B 1 1   1   1
Fogalaki 2   12  
Motu loa 1   23  4
Motu loa 2   11  13  4
Motu loa 1     12
Motu loa 2   18    12

The table tells us that the yellow crazy ant is the most abundant ant in Atafu and Fogalaki and and that the most yellow crazy ants are in plot A in Atafu. In places where yellow crazy ants are in low numbers or absent, more other ant species and insects are found.

You can also see from the table that the results for different traps are quite different and shows the reason for having replication. For example species 5 was only found in Motu loa because it is not present in the islets. This is an example of a biased result.

Results of pitfall traps may also be biased if they are placed near an active nest. Also for this reason the results for ants that nest in the ground aren't directly comparable with those ants that live in trees.

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Pitfall traps for tree dwelling ants

We haven't used these methods and don't know anyone who has used them, but we have included them for completeness. If you have used this method can you please let us know if it is useful and if we have described the method adequately.

Site selection and replication are the same as for ground pitfall trapping.

Pitfall trapping for tree dwelling species is quite difficult and usually involves the addition of an attractive lure to the preservative or trapping solution. Three methods for tree sampling are presented below. 

Basic tree pitfall traps

These traps use modified 20 ml plastic pottles which have lids. Cut a 1 cm hole in the lid of each 20 ml plastic pottles.

The traps use two flavours of liquid attractant lure:

  1. a 10% sugar solution
  2. a protein broth made by boiling either chicken or oily fish in water with some vegetable oil, then straining the liquid off to ensure it has no fragments of meat in it (as these will rot and be a health hazard)

You can add a few drops of unscented soap to each solution, which will break the surface tension of the liquid and make sure the ants drown.

Remove the lid and fill the container with either the sugar solution or protein broth.

Repeat this process so that half your pots contain sugar and the other half contain protein broth.

Place one of each lure type on trees at least 2 m above the ground, making sure that the lid of the pot is in contact with the trunk of the tree.

The pots can be secured in any vines growing on the tree or stuck on to the tree with a strap that is stapled to the bark.

Leave the traps in place for at least 24 hours then collect them.

The contents of the traps will need to be transferred to new jars. The trapping solutions must be drained off and (preferably) replaced with a preservative solution (e.g. ethanol).

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Canopy pitfall traps

The basic tree pitfall trap design can be modified for traps higher in the canopy by hanging the traps over upper limbs.

A pair of pots (one of each flavour) are fitted into a linen strap that is long enough to reach into the canopy, over a tree limb and back down to the ground.

Each end of the strap has a line attached to it. The end of one line is weighted, and this weighted end is thrown over an upper tree limb.

The strap and traps are then hauled over the limb and the lines are secured to the ground to make sure the strap cannot fall off.

Again, the assembly is left in place for at least 24 hours.

Alternative canopy pitfall trap

An alternative canopy pitfall trap uses the bottom third of a 2 L soda bottle with a plastic cup glued into the centre of it.

Essentially this creates a central lure surrounded by a “moat”, which the ants drown whilst trying to get to the lure.

The bait is placed in the cup in the centre of the cut down bottle.

Something with a strong smell like fish or honey (or a strong sugar solution) is good.

Four holes are punched in the upper lip of the cut down soda bottle, through which string is tied to make a pair of handles.

Water, with a small amount of detergent is added to the soda bottle making sure that none gets into the cup.

A weighted line is thrown over the upper limb of a tree. When it is over the limb of the tree it is pulled back so that it is close to the trunk end of the limb. The free end of the line is tied through the handles and the assembly is hauled up the tree.

Information sources

Agosti, Majer, Alonso, Schultz (eds). 2000. Ants: standard methods for measuring and monitoring biodiversity. Biological Diversity Handbook Series. Smithsonian Institution Press, Washington DC

Burne, Barbieri, Gruber. 2015-2019. Management Plan Atafu, Tokelau. Pacific Biosecurity Management Plan

Kaspari. 2000. Do imported fire ants impact canopy arthropods? Evidence from simple arboreal pitfall traps. Southwestern Naturalist 45(2):118-122

Oliveira-Santos, Loyola, Vargas. 2009. Armadilhas de dossel: uma técnica para amostrar formigas no estrato vertical de florestas [Canopy traps: a technique for sampling arboreal ants in forest vertical strata]. Neotropical Entomology 38(5):691-694

Samson, Rickart, Gonzales. 1997. Ant diversity and abundance along an elevational gradient in the Philippines. Biotropica 29(3):349-363

content reviewed by Phil Lester August 2016

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