Years ago an Italian colleague of mine, with great personal flare and espresso-enhanced gesticulations, gave me a back-of-the-napkin explanation of how to set long-term (4-6months), buried and baited traps for carabids. Europeans have been this doing for some time (e.g. see the paper by Gilgado et al. (2011) and the included references), but to my knowledge, no one in North America has done it. If you have, or know someone that has, I am very interested to know the results.
The basic idea is that there are species of carabids that are uncommonly found in leaf-litter, only found under deep-set large rocks or only occasionally found in caves, which are actually more common than they seem. The majority of active adults are moving through the spaces in the soil and rock (i.e. the mesovoid shallow substratum) and are never encountered. This would include some species are in Trechini and Bembidiini. California has scattered records of tiny (around 1-2mm), blind, flightless, pale anilline carabids, many that are undescribed species (D. Kavanaugh, pers. comm.). I have seen one specimen labeled Anillaspis explanata from about 9km NW of Portola Redwoods State Park, taken in a Redwood leaf-litter sample, but have not collected any myself.
I decided to put out some buried traps in California to see what I can get. Given the record from near Portola Redwoods S.P. and my ongoing work there, it seemed a good place to start.
My basic trap has the following components as show in the image: 1- Large outer jar, this one is about a 1.5L with a 10cm wide mouth. The preservative goes here. 2- Heavy monofilament fishing line tied to the top of the jar and bit of flagging. This is what sticks up on the surface so you can find the trap later. 3- Hardware cloth with about 5mm mesh, cut in a circle to fit 4– the jar lid with the center cut out. 5- Canning jar that will be the bait container and its ring lid. 6- Very fine mesh material. This is a scrap of nylon net mesh. 7- A scrap of widow screening. The mesh and screen let the scent of the bait out but keep the beetles from getting into the rotten goo bait. I used tin snips to cut the hardware cloth and a wood burning engraver to cut the plastic lid.
I then put 3-4cm of propylene glycol and a couple tablespoons of rock salt in the bottom of the large jar to act as preservatives (no DNA from these specimens). Rotten chicken liver or spoiled brie cheese is placed in the small jar. The small jar is placed in the large one. I also have a shield that goes above the trap when buried to deflect water that might go straight down through the soil and fill the trap.
Next I got some students wielding shovels, picks and post-hole diggers to hike all the gear out into the redwood and Douglas fir forests of Portola Redwoods State Park and dig them in.
We dug taps in about 0.5-1.0m deep. Put soil right up to the lip of the large jar (like a pitfall trap on the surface would be), placed a couple small rocks or roots on top the hardware cloth, put the shield on top of those (this leaves a little space for entry into the trap) and buried the whole thing, leaving only the flag tied to the line showing.
I will return in May to see if any of beetles have found their way into the traps. I’ll let you know what I get.
Now that I have the gear and routine worked out, I hope to put out more traps in different habitats in the state to see what sort of beetle secrets California has for us.
Gilgado, José D., et al. “Description of the first larval instar of Broscus crassimargo Wollaston, 1865 (Carabidae: Broscini) and notes about the presence of the species in the mesovoid shallow substratum of La Gomera (Canary Islands, Spain).” Entomologica Fennica (2011).