As the spring insect collecting season gears up I decided to sing the praises of one of our favorite low-tech, low-cost collecting methods – the Trunk Trap (or as we call it in the lab – the “Eleanor Trunk Trap”). There are almost as many insect collecting devices as there are insect collectors, but relatively few of these traps or extractors are truly original designs; most are modifications on existing traps or extractors. Our trunk traps are a case in point; but they do their job very well.
Lots of insects live on tree trunks, running up and down the bark, hunting or feeding in the cracks and crevices, dispersing out from tree wounds and tree holes. The problem is that many standard insect collecting methods do not target this microhabitat. Some entomologists – mostly European, mostly interested in beetles – developed a series of small trap designs usually with vertical baffles or windows, that proved effective in collecting beetles on or near tree trunks or branches. Many of these designs, however, had only a small part of the trap actually in contact with the trunk.
Eleanor Fast was a grad student in my lab who was interested in fly diversity in deciduous forests, especially flies associated with big, old trees. In 2001, Eleanor came up with the simple idea to dispense with baffles, windows, etc. and get more of the rim of the trap right up against the tree trunk. The simplest way to do this was to use the top half of a 2 litre soft drink bottle, turned upside down and tied tightly to the trunk with string. Eleanor bought a roll of string and all her friends were immediately requested to start drinking more soda! Empty bottles started to fill her office, an old pair of lab scissors followed, and the trunk trap (at least, Eleanor’s version) was born.
To empty the trap, Eleanor simply held a container beneath the bottle, unscrewed the lid, and let the contents drain out. She filtered out the insects and replaced the liquid in the trap. Subsequent versions of the trap built in the lab have small bottles glued below the funnel, but the original design worked perfectly.
Eleanor’s M.Sc. project was an inventory of forest flies at the Mont Saint-Hilaire Biosphere Reserve in southern Quebec (see Fast & Wheeler 2004 in Publications) and her sampling design included standard methods (Malaise trap, yellow pan traps, sweeping) as well as several trunk traps tied to American Beech and Sugar Maple trees in the old-growth forest on the mountain. The results from the trunk traps were surprising.
We originally expected to get a few specimens of flies known to run up and down tree trunks (some Dolichopodidae, a few other Empidoidea, some Phoridae), but in the end, the trunk traps yielded up more than 4000 flies, in almost 100 species, including some groups usually considered “rare”. These rarities included almost all the specimens she collected in the families Aulacigastridae, Periscelididae and Odiniidae (very few of these in Malaise or pan traps). Eleanor also collected several specimens of a milichiid fly which later proved to be a new species, Neophyllomyza gaulti, closely associated with rotting logs in mature forests (see Brochu & Wheeler 2009 under Publications).
The key to the success of this simple design is probably the fact that the edge of the bottle is pulled tightly against the trunk, so that a big part of the upper rim is right against the bark. This means that there is more opportunity for insects walking down the trunk to be captured. This trunk trap has become one of our favorites in collecting forest Diptera and we’ve used it in a number of student projects since 2001.
A ball of string and a few empty soda bottles. A very small investment for big returns. Give it a try.