How do you monitor which species live in an area? In addition to traditional ecological tools such as camera traps, researchers have reported new methods in recent years that allow them to detect minute traces of DNA known as environmental DNA, or eDNA, that animals leave behind in water and even air. In a study published June 15 in Biology Letters, a group reports picking up eDNA from a new source: dried plant material. The team purchased tea from grocery stores, and were able to detect hundreds of species of arthropods in just one bag.
We asked study coauthor Henrick Krehenwinkel, an ecological geneticist at Trier University in Germany who focuses on the ways in which arthropod communities have changed over time due to human influence, to spill the tea about why his group decided to use eDNA to investigate which critters have been munching on plants.
TS: Why did you decide in this case to focus on tea?
Henrik Krehenwinkel: We need [a] time series to understand how insects have changed. When insect decline studies were first published, a lot of people complained [that] there is no real long-term data.
We have a specimen bank here in Trier. They’re collecting leaves from different trees in Germany. They’ve been doing this for 35 years; they go to all kinds of different ecosystems. . . . And what I asked myself is, ‘Couldn’t you also monitor the DNA of the insects which have lived on this leaf?’ . . . We basically did a test experiment where we took these samples, which are frozen in liquid nitrogen, so they’re perfectly stored for DNA preservation . . . and isolated DNA from them, and reconstructed arthropod communities. This is actually another study which is currently in review, where we have basically reconstructed insect community change in German forest ecosystems over the past 35 years.
So we can extract eDNA from a perfectly frozen leaf. . . . What I asked myself is, “Can you also use other substrates to basically extract the DNA from arthropods?” And is the DNA still stable in other types of substrates? . . . Plant collections in museums, could they actually be useful to understand how insect communities have changed? . . . There are studies saying that . . . if an insect bites into a leaf, it will leave a DNA trace; a little bit of saliva is enough. It’s basically like [how] the criminal breaking into your house, touching your window, will leave their DNA; the insect will leave its DNA when it bites into the leaf. And there are studies saying that this DNA is not very stable, it will be quickly degraded by UV light or washed away by rain. But I was thinking in an herbarium record, the DNA is stored dry and dark, which [are] actually ideal conditions to maintain it.
Before we started working on herbaria records, we thought we should try something which is kind of comparable to herbarium records. . . . Structure-wise, [tea is] very similar to herbarium record. It’s basically a dried plant which is stored dark and dry. . . . And the DNA should be very stable.
It’s all driven by our hope to understand insect community change and being able to find new substrates which allow us to travel back in time. . . . [Y]ou can collect a plant in the field, basically a flower. And you can dry this flower just using silica gel. . . . It’s a substance which is completely harmless, but it’s extremely hydrophilic. . . . If you, for example, put a flower into a little envelope, and then you put it in a Ziploc freezer bag together with a little bit of silica gel, within one day approximately, the flower will be completely dry. . . . [A]nd we could in theory even store them at room temperature, we wouldn’t have to worry to put it all in liquid nitrogen or to wash the plant right away . . . you don’t have to carry water in the field, all you need is a little bit of silica gel, an envelope, and a Ziploc bag.
Another attractive side effect is that what’s very interesting for us ecologists is not only who is at a site, so how many insect species are at a site, . . . but we also want to know how do these insects interact and what do they eat. For example, we know that many insect species are very specific, only living on a certain plant, and when this plant disappears, the insect disappears. . . . Surprisingly, we know very little about these interactions, we know very little about what insect is limited to a certain plant species. We know this pretty well for pest species, but we do not know this pretty well for many other species of insects. . . . And this is a way of very quickly finding this out by basically sampling plant material and being able to associate the insects living on the plant.
TS: Was there anything about the results of this study that surprised you?
HK: What really surprised me was the high diversity we detected. . . . We took one tea bag, and . . . I think it was from 100 [or] 150 milligrams of dried plant material, we extracted DNA. And we found in green tea up to 400 species of insects in a single tea bag. . . . That really surprised me. And the reason probably is that this tea, it’s ground to a relatively fine powder. So the eDNA [from all parts of the tea field] gets distributed.
TS: As far as applying this to herbaria samples, would you need just a relatively small piece of that sample, or could it be an issue that these are rare and very old samples, and you don’t want to be grinding up a big chunk of them.
HK: We’ve been thinking about this, and there’s two options. One is to just very carefully treat the herbarium sample. We’re now testing if you can also just carefully wash the sample, for example, and kind of wash off the traces which are stuck to the sample.
Then of course, there’s herbaria where they’re actually happy if you do something with them. [H]ere at this university, we have a retired botany professor, and she has very large herbaria she has collected during her tenure. . . . They don’t have a huge scientific value for her, and she would be fine if I grind them up. . . . We’re just testing this, so I cannot give you any results on this yet, but it looks like we are actually able to extract insect DNA out of this as well. . . . And then move back to that same place—she has exact collection sites—I just drive back there, collect the same plant again, and then I can compare how was the insect community 50 years ago when she collected it or 30 years ago when she collected again, and then compare it to how is the insect community on that plant today.
But of course, generally these collections are very precious and we are developing methods to carefully extract DNA from this without damaging the specimen. This is something we’re just starting now. Seeing how well it worked with tea, I’m now confident that we could also move into other samples like these herbaria.
TS: Are you a tea drinker yourself?
HK: I drink coffee actually. . . . And I fear coffee probably is not well suited for it because coffee is roasted. And what DNA really doesn’t like is being heated up to a very high temperature for a long time . . . . We have not tried it yet, but I fear coffee is probably not the best choice for this kind of experiment.
Editor’s note: This interview has been edited for brevity.