There are bugs in your cup of tea. Not whole bugs (hopefully not). But the essence of a hundred species of insects can be found in that tea bag you buy from the store. Even if you have been sipping tea for decades, I don’t think you need to worry about turning into Jeff Goldblum from The Fly.
Edmond Locard, the father of forensic science, wrote in his 1939 manual on police techniques that it is impossible for criminals to act without leaving traces of having been on the scene. Every contact, it is often said, leaves a trace.
The insects and spiders that buzz about and skitter around the fields of tea in the wild, taking a bite here and there and leaving behind waste—while doing nothing criminal—do leave a trace. And scientists can now detect that trace.
It’s called environmental DNA, and it’s not just on your tea leaves. It’s also in the air you breathe.
I can feel DNA in the air tonight
It began with a gripe. We are often told that certain insect populations have been declining over time, but where is the long-term data? It can be challenging to compare present-day data to the situation twenty, thirty, fifty years ago. Time travel is unlikely to remedy the situation, but what if the data we seek from the past is already around us, frozen in time?
This was the impetus for studying tea leaves, according to Henrik Krehenwinkel, an assistant professor at the Department for Biogeography at Trier University in Germany. Insects leave marks on the plants they interact with, specifically their DNA. Exposed to the elements, DNA can quickly degrade. It is deformed by the Sun’s ultraviolet rays and chewed up by enzymes. But in plant specimens that were carefully collected and kept in dry and cool storage, this trace has a better chance at surviving. A museum’s plant collection fits the bill, but so do tea leaves.
Professor Krehenwinkel’s investigation of tea leaves is part of a larger and quite recent discipline known as the study of environmental DNA, or eDNA for short. With eDNA, the sample does not come from the organism, but rather from the environment it occupies, like a criminal’s genetic fingerprint inadvertently left at the scene of their deed. Nowadays, scientists find the DNA traces of wildlife in water and snow. Mammals and insects visit rivers, streams, and shores, shedding the molecule of life in their saliva, feces, urine, and multifarious bodily secretions. They contaminate their surroundings with crumbs that can be traced back to them.
We were not always capable of detecting eDNA. But the same molecular tools that have allowed us to spot the SARS-CoV-2 coronavirus up someone’s nose and to identify its precise strain have empowered ecologists to detect traces of wildlife in water. First, DNA is extracted from the sample. Then a section of it is amplified via the polymerase chain reaction (PCR), which acts like a photocopy machine churning out stacks of duplicates of a specific page. Finally, the amplified DNA is sequenced, and the letter-by-letter sequence is matched to a virtual database to identify the culprit.
In the Amazon, the presence of anteaters and river dolphins was observed by finding their DNA in the water. In the United Kingdom, eDNA tattled on some water voles that had never been documented in a particular area. The furry-tailed water vole, looking not unlike a large guinea pig, was later caught in that spot with a camera trap, thus confirming the results of the eDNA test.
But eDNA doesn’t just drift in water; it floats too. Two independent teams, without knowledge of each other’s work, decided to test whether or not they could detect animal DNA in the air inside of a zoo, one in Denmark and the other in England. They set up traps with vacuums that would force the air through a filter. What was trapped was amplified via PCR and sequenced, and lo and behold, the researchers not only detected many of the species their respective zoos featured, but also DNA from the food being fed to these animals. It’s not just the coronavirus that is airborne, but our very DNA is shed and drifts, aloft, ready to be detected by ever-more sensitive assays.
Reading the tea leaves
Which brings us back to tea.
Using this technology, the German team tested green tea, dandelion tea, samples from European beech trees, chamomile, mint, and parsley to see which predator, herbivore, pest, and casual visitor might have come into contact with them. From these plant specimens, they recovered DNA from a total of 1,279 species, with the majority seemingly having been present before harvest and a small number of species typically tagging along during storage.
What surprised the scientists was the large diversity of species they observed. “We found in green tea,” Professor Krehenwinkel, the lead author on the study, told The Scientist, “up to 400 species of insects in a single tea bag.” That’s a lot of bugs in your teacup.
Besides satiating our curiosity, sampling our surroundings for traces of the past carries within it much potential. When testing plants that were collected a long time ago, it is a window into bygone decades, a way to reconstruct the insect diversity of yesteryear to compare it to today’s. But even when testing recent samples, it has a utility. The monitoring of arthropods—invertebrates like spiders, ticks, centipedes, and insects, with segmented bodies and an exoskeleton—typically involves killing them. Malaise traps are tents that draw the arthropod up into a collection vessel that exterminates and preserves its victim, while pitfall traps use buried containers filled with a powerful chemical preservative. By contrast, environmental DNA collected off of plants is cruelty-free.
Moreover, the pests that hitch a ride onto herbs and tea leaves have an origin. By carefully testing their DNA, scientists could potentially geolocate these herbs and leaves through inference. If the insect hitchhikers came from China but the seller is telling you the plants came directly from the Pacific Northwest, something is amiss. And eDNA monitoring holds the promise to act as an early detection system for pest outbreaks, detecting their presence in stored plants before they can cause real problems.
Environmental DNA is one more instrument in a field scientist’s toolbox to get an accurate picture of an area’s wildlife and to monitor rare, endangered, or camera-shy species. Camera traps are useful but they have blind spots. EDNA can provide additional sight. By sampling the environment, a biological kiss between fauna and flora can be revealed.
While we’re justified in our excitement for this new technology, there are still puzzling problems to overcome. For example, where are the tigers? One of the teams that sniffed out a zoo’s air for DNA could not detect any tiger DNA in their samples, despite the fact that the zoo did house the felines. Do tigers shed less DNA than birds? Was there an issue with sample collection or DNA sequencing? Is this an issue likely to impact other species?
And beyond this question of absence is another one, that of abundance. How do we use eDNA to put a number on how many crickets existed in a field when a plant was collected? Much like the amounts of plant chemicals vary with seasons, soil composition, and a myriad other factors, the pipeline from an insect traipsing around a field and a scientist detecting its DNA when sequencing a plant sample is likely to be influenced by a number of seasonal variables.
Our eyes have been opened to the presence of DNA being shed by animals into our environment, but this isn’t something we can use well at the moment.
In the meantime, when you brew your next cup of tea and take a sip from it, do know that beyond the tannins, flavanols and DNA of the Camellia sinensis plant itself, you are also ingesting the As, Ts, Cs, and Gs of hundreds of little critters.
You are tasting an ecosystem suspended in time.
- Animals, including insects, shed their DNA in their environment and this genetic material, referred to as environmental DNA or eDNA, can now be detected by scientists in water, air, and even in plant samples
- Scientists from Germany recently tested a number of tree and plant specimens, including store-bought green tea bags, and detected eDNA from 1,279 species, including up to 400 insect species in a single teabag
- This type of analysis, while still in its infancy, has the potential to detect storage pests before they become a problem, to help scientists monitor camera-shy animals in the wild, and to paint a picture of past biodiversity by testing plants that were collected many years ago