My subterranean pitfall traps have turned up some very rarely seen beetles in recent years, including the first British specimens of Oxylaemus cylindricus since c.1900 as well as Oxylaemus variolosus, Medon dilutus, Medon castaneus, Trichonyx sulcicollis and Aeletes atomarius. A few people have asked me for more information about the traps. So this is a guide to building and using subterranean pitfall traps for beetles.
The subterranean pitfall traps are based on a section of rainwater downpipe and are a modified version of an original design by John Owen (1995). The trap is buried with its top flush with the ground surface and capped so that invertebrates can only enter through the mesh sides of the trap. The screw thread on the collecting bottle allows it to be set in place at the bottom of the pipe, retrieved and replaced without having to dig up the trap, using a screw-cap fixed to the end of a long rod (the servicing rod).
The crucial requirement to build a successful trap is to find screw-top poly-bottles of a diameter which is just less than the internal diameter of rainwater downpipe. By carefully wrapping insulating tape round and round below the shoulder of the bottle, its diameter can be built up layer by layer until it just fits into the downpipe with minimal gap. Then cut three holes into the shoulder of the bottle.
My traps are made from a single 50 cm length of downpipe. I cut three oval holes in the sides in order to trap beetles active in the soil between 5 and 35 cm depth. My collecting bottles are 13 cm tall, so there must be at least 13 cm of intact downpipe below the bottom edge of the hole to house the collecting bottle. I cut the holes out of the downpipe using a jigsaw and in the example shown, I have marked the areas to be cut using red insulating tape. The three remaining vertical spans of pipe wall that remain, each the width of a piece of insulating tape, are enough to keep the structure rigid.
The mesh is then wrapped round the pipe and held in place just by a couple of cable ties, pulled tight, at top and bottom. It is possible for the mesh to move up and down over the pipe but in practice, this is not a problem. I have used traps with broader gauge mesh than illustrated: they catch larger beetles such as Carabus species, Dorcus parallelepipedus, etc. but let more soil fall in meaning dirtier samples.
I tend to use neat vehicle antifreeze or propylene glycol as the trapping fluid. Owen (1995) used a mixture of sherry and vinegar. Unlike surface pitfall traps which need to be serviced every few weeks if they are not to become dried out or flooded, subterranean traps seem to be little affected by the weather and can be comfortably left for a couple of months in the summer between servicing visits.
If I made any more traps, I would definitely make them shorter, maybe 40 cm long. I have often found that digging a 50 cm deep hole at the roots of a tree is a long, sweaty and blistering job and on several occasions I have had to give up on about the 6th attempt after hitting yet another solid root, immovable rock or clay pan.
Apart from a few early trials in the garden (notable for finding Oxyomus sylvestris (Scarabaeidae) near the compost heap) I have only used subterranean pitfall traps to sample at the roots of dead or decaying veteran trees. As I’ve been targeting saproxylic beetles, my tactic has been to try and set the trap right up against decaying roots, preferably soft, sappy and with signs of fungal decay. As I’m digging the hole, I’ll separate the spoil into soil and woody material, so that when I backfill, I make sure that any bits of rotten root that I’ve dug through get placed up against the mesh. I backfill the hole quite loosely so that there will be plenty of room for invertebrates to move through the soil.
I then cap the top of the pipe, put a log or stone over the cap, bury it out of sight and take a few photos of the spot so I can find it again.
The samples are inevitably quite dirty, especially the first, but I’m often surprised at how little soil there is in the samples by the third or fourth visit. But at their worst, it can take a couple of hours per sample to separate the beetles from the dirt. The identification process can also be very time-consuming but in a good way – don’t run subterranean pitfall traps if you don’t want to have to identify loads of beetles that you’ve never seen before!
There is often a substantial bycatch of woodlice, millipedes, centipedes, spiders and flies, amongst other groups. I’ve yet to find anything amongst the woodlice, millipedes and centipedes that couldn’t easily be found without subterranean trapping. I’m more optimistic that these traps could turn up some rarely seen flies but although a few of my subterranean trap samples have been looked at by dipterists, there’s been nothing noteworthy so far.
Owen, J.A. (1995). A pitfall trap for repetitive sampling of hypogean arthropod faunas. Entomologist’s record and journal of variation, 107, 225 – 228.