How do you monitor what species live in an area? In addition to traditional ecological tools such as camera traps, researchers in recent years have reported new methods that allow them to detect tiny traces of DNA called environmental DNA, or eDNA, that animals leave in water and even in the air. In a study published on June 15 in Biology Letters, a group reports recovering eDNA from a new source: dried plant material. The team bought tea from grocery stores and were able to detect hundreds of arthropod species in a single bag.
We asked study co-author Henrick Krehenwinkel, an ecological geneticist at the University of Trier in Germany who focuses on how arthropod communities have changed over time due to human influence. , to spill the tea on why his group decided to use eDNA to investigate the creatures that munched 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 the insect decline studies were first published, many people complained [that] there are no real long-term data.
See “Germany sees a drastic decrease in insects”
We have a specimen bank here in Trier. They collect leaves from different trees in Germany. They’ve been doing this for 35 years; they go into all kinds of different ecosystems. . . . And what I wondered was, “Couldn’t you also monitor the DNA of the insects that lived on this leaf?” . . . We basically did a test experiment where we took these samples, which are frozen in liquid nitrogen, so that they are perfectly stored for DNA preservation. . . and isolated their DNA, and reconstructed arthropod communities. This is actually another study currently under review, in which we have essentially reconstructed the evolution of insect communities in German forest ecosystems over the past 35 years.
Thus, we can extract eDNA from a perfectly frozen leaf. . . . What I wondered was, “Can you also use other substrates to essentially extract DNA from arthropods?” And is DNA still stable in other types of substrates? . . . Could plant collections in museums really be useful for understanding how insect communities have changed? . . . There are studies that say that. . . if an insect bites into a leaf, it will leave a trace of DNA; a little saliva is enough. It’s basically like [how] the criminal breaking into your house, touching your window, will leave his DNA; the insect will leave its DNA when it bites into the leaf. And there are studies that say this DNA is not very stable, it will be quickly degraded by UV light or washed away by rain. But I thought that in a herbarium, the DNA is stored dry and in the dark, which [are] in fact ideal conditions to maintain it.
Before we start working on herbarium records, we thought we should try something that’s somewhat comparable to herbarium records. . . . On the structural side, [tea is] very similar to the herbarium record. It is basically a dried plant that is stored dark and dry. . . . And the DNA must be very stable.
Everything is driven by our hope to understand the evolution of insect communities and to be able to find new substrates that allow us to travel through time. . . . You can harvest a plant from the field, basically a flower. And you can dry this flower just by using silica gel. . . . It is a completely harmless, but extremely hydrophilic substance. . . . If, for example, you put a flower in a small envelope and then put it in a Ziploc freezer bag with a little silica gel, in about a day the flower will be completely dry. . . . And we could theoretically even keep them at room temperature, we wouldn’t have to worry about putting everything in liquid nitrogen or washing the plant right away. . . you don’t need to carry water in the field, all you need is some silica gel, an envelope and a Ziploc bag.
Another attractive side effect is that what is very interesting to us ecologists is not just who is at a site, so how many species of insects are at a site, . . . but we also want to know how these insects interact and what do they eat. For example, we know that many species of insects are very specific, living only on a certain plant, and when that plant disappears, the insect disappears. . . . Surprisingly, we know very little about these interactions, we know very little about which insect is restricted to a certain plant species. We know this well enough for pest species, but we don’t know it well enough for many other species of insects. . . . And it is a way to discover it very quickly by essentially taking 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?
Hong Kong: What really surprised me was the great diversity we detected. . . . We took a tea bag, and. . . I think it was from 100 [or] 150 milligrams of dried plant material, we extracted DNA. And we have found in green tea up to 400 species of insects in a single tea bag. . . . It really surprised me. And the reason is probably that this tea is ground into a relatively fine powder. So the eDNA [from all parts of the tea field] is distributed.
TS: In terms of applying this to herbarium samples, would you need a relatively small piece of this sample, or could it be a problem that these are rare and very old ones, and you don’t want to grind up a big chunk of them.
Hong Kong: We have thought about it, and there are two options. The first is to treat the herbarium sample very carefully. We are now testing whether you can also wash the sample thoroughly, for example, and somehow wash off the traces that are stuck to the sample.
Then, of course, there are seagrass where they are really happy if you do something with them. [H]Here at this university we have a retired professor of botany, and she has very large herbaria that she collected during her tenure. . . . They don’t have much scientific value to her, and she’d be fine if I crushed them. . . . We’re just testing this, so I can’t give you any results on that yet, but it looks like we’re able to extract insect DNA as well. . . . And then I go back to the same place – she has exact collection sites – I go back there, collect the same plant again, then I can compare what the insect community was like 50 years ago when she took it collected or 30 years ago when it collected again, then compare it to the current state of the insect community on that plant.
But of course, usually these collections are very valuable and we are developing methods to carefully extract the DNA from them without damaging the specimen. It’s something we’re just starting now. Seeing how well it worked with the tea, I’m now convinced that we could move on to other samples like these herbals as well.
TS: Are you a tea drinker yourself?
Hong Kong: I actually drink coffee. . . . And I’m afraid the coffee is probably not suitable because the coffee is roasted. And what DNA really doesn’t like is being heated at a very high temperature for a long time. . . . We haven’t tried it yet, but I’m afraid the cafe probably isn’t the best choice for this kind of experience.
Editor’s Note: This interview has been edited for brevity.