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Flotsam harvesting of detritus samples

Flotsam harvesting of detritus samples
After my abundantly successful 2007 experience collecting arthropods from flotsam in a catchment basin in New Hampshire I began to think of how I could employ a similar strategy in a neotropical setting since I was planning a move to Ecuador where I now live. I finally settled on an approach in May of this year that I put together in late June.

I had been sifting rotting bamboo leaf litter to see what I could find living in it with a special interest in beetles. My results were certainly encouraging but the time requirement was enormous. I filled the top part of my sifter about half full, put the lid on, and shook the contents vigorously for a minute or two so that the smaller particles and any small arthropods would fall through the wire mesh into the bottom part. Then I would sit staring into the fine debris in the bottom part watching for telltale movement. This phase consumed 10-20 minutes since many arthropods, especially beetles, play dead after being rudely shaken and don’t “come alive” for many minutes. Progress was therefore quite slow.

Many an entomologist at this point would think of using a Berlese funnel with a hot light bulb shining inches overhead to drive the detritus denizens downward into a collecting container, often filled with alcohol in which they would be killed indiscriminately. However, in my solar-powered camp, a hot light bulb would consume a big part of my electricity production. Besides, I knew of a faster way. Instead of using heat and dryness to drive small arthropods downward I could use trickle flooding to drive them slowly upward onto small broken-up pieces of plastic foam. Once floating, the white foam chunks covered with arthropods could then be removed and the now easily seen arthropods selectively collected via aspirator (aka pooter), all others being liberated.

On my most recent twice-monthly shopping trip I bought two small trash barrels and a nylon sieve for $9.00 a ¾-inch plastic ball valve for 85 cents, and a double-gasket plastic tank fitting for $1.50 (prices may vary elsewhere of course). I also bought a meter of wire fencing. I took one of the barrels, the pipe fittings, and the wire fencing to a workshop where we melted a hole in the bottom of the barrel with a torch-heated chunk of metal pipe. That was to allow the tank fitting and the valve to point straight down from the center of the bottom of the barrel. Next I had them fashion a disc from the wire fencing to rest on top of the load of rotting leaf litter in my other barrel. Its purpose was to keep as little of the leaf litter from floating as possible while allowing arthropods to escape upward. They did a bang-up job, first welding some heavy wire into a circle, then wrapping the cut wire ends around the hoop with pliers. I paid them $10. (Labor is cheap in Ecuador.)

Once home with my components I needed to build a frame to support the barrels, one above the other. I cut up some small logs into tall posts with my chainsaw, sharpening their ends. These I plunged into the soil repeatedly till I had four of them firmly planted in a rectangle. Next I made two level platforms, one high, the other low, using some boards that I cut with my chainsaw. I nailed them onto the uprights I had just installed. The top platform had a space between two of its boards to allow the tank fitting and ball valve to hang down.

I filled the bottom trash barrel with rotting leaf litter up to a certain point, then placed the wire disc on top and covered with broken-up beadboard chunks (often incorrectly referred to as Styrofoam). Next I draped a piece of sheer curtain fabric over the top of this barrel and tied with a double loop of cord to prevent escape by any of the arthropods. I left a big fold in the material that would allow me access to the flotsam but this too I secured with a second knot in my cord. That way I could open and close it without the rest of the material coming undone. The final step was to fill the top tank with water from my nearby roof-runoff catchment tub and open the valve just barely enough to permit a medium-rate dripping into the lower trash barrel, thus giving arthropods plenty of time to climb higher in the debris. I put a lid on the top barrel to prevent insects from getting into it.

The entire setup is actually sheltered under a plastic roof I erected for a UV light and moth sheet but if it was a stand-alone structure I would have made a separate roof over it to keep the rain from over-filling the bottom barrel. (I live in a very rainy cloud forest region.)

Though based on my catchment basin experience in New Hampshire and my sifting results here in Ecuador, my plan was admittedly experimental. I had reason to expect that the floating plastic foam chunks would be crawling with thousands of arthropods, including mites, springtails, millipedes, centipedes, pseudoscorpions (I’ve never found a real scorpion in my region), harvestmen, spiders galore, sowbugs, symphylans, diplurans, nicoletids, and a host of insects that would include roaches, true bugs, tiny wingless phorid flies, maggots of various larger flies, tiny parasitoid wasps, ants, termites, crickets, a few caterpillars, and hundreds of beetles, both adults and larvae, including ptiliids, histerids, hydrophilids, endomychids, dryopids (the terrestrial ones), erotylids, anamorphids, weevils, cantharids (their larvae anyway), tenebrionids, carabids, a glut of staphylinids, nitidulids, biphyllids, ptilodactylids, and no doubt members of a few other families. Non-arthropods would include snails, slugs, land planaria, and worms.

By trickle-flooding the live specimens upward I figured to snare some arthropods, beetles especially, that either remained in the upper part of my sifter or played dead longer than I was willing to wait for them to come alive. I would also avoid damaging specimens from the vigorous shaking of my sifter and would save an enormous amount of time that I could devote to other activities while the bottom barrel slowly filled.

(A word of caution: I used my bare hands to collect litter since at my elevation there are no venomous snakes and I have yet to see a scorpion in all my collecting activities. At lower elevations I would be using thick gloves and/or small gardening tools to gather leaf litter.)

So how did my results compare with my expectations?

My first result was more of a non-result. The top water-filled trash barrel that I had left dripping overnight at the rate of about one drop per second had stopped dripping. I lifted the lid to check the water level, concluding that the dripping had stopped soon after I left it. Obviously a small piece of debris had plugged the minute crack opening of the ball valve. I opened it wider to free the obstruction, then set it on about three drips per second. The lesson learned: debris-free water and periodic checking is required with this setup.

In short, the trial run was a success. I collected around 100 beetles plus a few other arthropods from my sample, including at least two new-for-me species of Cerylonidae, one new-for-me species of Byphyllidae, and an undescribed species of Discolomatidae I had previously found in one small area of maybe 20 square meters a few miles away. I scooped out three batches of my flotsam to examine in the bottom part of my sifter and sucked up what I wanted with my aspirator (aka pooter).

In the above photo you see the stand I made to support both upper and lower trash barrel in my flotsam harvesting setup.

Note to editors: Please do not treat these specimen photos as Bug Guide submissions as they are from outside the Bug Guide region and only included for interest in this collecting method.

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Amazing idea!
I will definitely replicate your slow trickle approach to surveying populations ;)

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