Our collection manager Zack and I have been talking about soil washing in Kansas for some time now. Soil washing, for those of you who don’t know, is a technique used to collect insects (in my case) that are subterranean.
In staphylinids, two lineages, Leptotyphlinae and Mayetiini (Pselaphinae) are exceptional in having taken this lifestyle to its extreme and have appreciably diversified in doing so. The two are surprisingly convergent in morphology, although they occupy disparate regions of the rove beetle tree of life.
Zack had heard a rumor that a leptotyphline, one of these subterranean staphylinids, had been collected from a root-ball in a Kansas prairie. Rumor passes down to me, and this rumor was what fueled our motivation to put into action, what is arguably the most tedious method of collecting.
Step 1. You dig. I don’t know, pick a spot, any spot. We chose a variety.
Me, digging for, well… soil, out in Kansas prairie. Photo courtesy of Zack.
Step 2. For this step, you literally wash the soil. I dumped the soil etc. into trash bin and added water with a hose. After a reasonable amount of water had covered the soil underneath, I sloshed the soil around and skimmed off anything that floated to the surface (this is all organic material, including bugs).
Washing some soil.
Step 3. I then wrapped any organic material I skimmed from the surface of the water into a bundle, using cheesecloth. I then wrung out this organic bundle of any excess water, wrapped it again, this time in paper towels to further dehydrate, and stored them in labeled baggies until Berlese time.
A bundle of organic stuff. The samples can be stored like this for about a week, so you can transport them back from the field if you’re far from home.
Step 4. Berlese time. It only took about a day, the stuff wasn’t very dense.
Berlese! A little makeshift spot for this particular project.
Step 5. Sort.
Mayetia peasei are easily packed into a 1 cm square.
To some extent, I think I speak for both Zack and I when I say we had low expectations. This added to our surprise when we found close to 20 individuals of a teeny-teeny staphylinid. We go back and forth for some time about whether it was a leptotyphline or Mayetia, finally settling on the later – I mean they’re so damn convergent-looking!
A little more digging around lead to a species identification, Mayetia pearsei. This species is presumably parthenogenic, argued from the fact that not a single male has been discovered, despite some serious effort. Other species in the genus are known to demonstrate extremely female-biased sex ratios.
Mayetia populations aren’t very dense and individuals are poor dispersers. If chances of meeting a mate are unlikely, this can fuel the establishment of parthenogenesis in a population. In some species where males are present in low frequency, they may play a dwindling role as a population marches on towards purely parthenogenic reproduction. Also possible, males are retained for periodic out-crossing, which theoretically can help avoid pitfalls like Muller’s ratchet and environmental instability. Whatever the case, it seems clear to me that this is an interesting system that deserves more attention; several obvious questions and solutions immediately pop in mind.