Reading Bug Tracks on Tea Leaves

From palm-reading to watching Fox News, humans throughout the ages have sought knowledge through some decidedly irrational means. But every now and then, superstition pays off.

For example, studying the pattern of coffee grounds in the bottom of one’s cup, a practice known as tasseomancy, will nearly always reveal that someone forgot to put a filter in the coffeemaker basket. And haruspicy, the study of the fresh entrails of a gutted animal, is consistently right in concluding the animal is dead.

The reading of tea leaves, a custom devised by 17th-century Europeans who got bored waiting for the Internet to be invented, has become quite popular among entomologists and other biologists in the past few years. It’s possible a few may secretly use this method to gain insight on their personal lives, but for the most part, tea leaves are now read by scientists to learn about insect behavior around the world.

It turns out that for any given tea leaf that ever had a beetle bite it, scale-insect suck sap from it, or planthopper poop upon it, traces of DNA from each and every visitor stay behind. Even after you’ve brewed a refreshing pitcher of iced tea, it’s possible to map out who else took a taste first. Recent breakthroughs in genetic sequencing allow for the detection of DNA remnants at extremely low levels. All it takes is a part of one sloughed-off arthropod cell on a leaf to identify its owner down to the species level.

This is called environmental deoxyribonucleic acid, or eDNA for short. A July 20th, 2022 Smithsonian article outlines how a team of scientists in Germany used eDNA to catalog more than 1,200 different species of arthropods on samples of parsley, mint, black tea, and chamomile. Before you panic, not all 1,200 critters are joining you in each cup of herbal or regular tea. Hundreds of plant specimens were tested across four continents to get this number. On average, “only” about 200 different insects and arachnids left their genetic calling-cards in each tea bag. This is actually a good thing, as it shows that our tea isn’t being doused with pesticides.

As noted in the Smithsonian article, eDNA is a powerful tool for monitoring insect populations, which are in freefall across the globe. In example, surveys done on nature preserves in Germany over the past twenty-seven years showed a 75% decline in the number of flying insects during that period. Similar findings are coming in from every continent. In addition, eDNA on tea leaves can help track the spread of invasive pests.

In light of this new study, other scientists hope to be able to old obtain specimens from botanical collections and museums. Comparing recent results with eDNA from plants plucked decades ago will give us a better understanding of trends in arthropod populations.

Closer to home, the Ontario Ministry of Natural Resources uses eDNA sampling to check for oak wilt, a devastating pathogen of oak trees first identified in the US. As early as 2018, oak wilt was known to be just a few kilometres from Ontario along significant stretches of its border with Michigan. Not surprisingly, in 2020, oak wilt eDNA was detected in Ontario. However, the first confirmed case of oak wilt in Canada has yet to be found.

In northern New York State, the St. Lawrence-Eastern Lake Ontario Partnership for Regional Invasive Species Management (SLELO PRISM) has been conducting eDNA surveys for invasive aquatic species in major tributaries of the St. Lawrence River. For the past several years, the Black, Oswegatchie, Grasse, and Raquette Rivers are being monitored for the presence of invasive fish such as the northern snakehead (Channa argus), a.k.a. Fishzilla. This über-predator would displace muskies and pike if it got established in the region, vacuuming up a great deal of the perch and bass along the way. Fishzilla is able to walk on land in search of new homes, meaning that it could conceivably shed DNA on tea leaves on its way to ravaging the local fishery.

Waters in the St. Lawrence River and eastern Lake Ontario near the US-Canada border are also checked for eDNA signatures left by silver and bighead carp, either of which could markedly degrade water quality. Hydrilla (Hydrilla verticillate), the most pernicious aquatic weed ever found in the US, is also on the eDNA watchlist. This invasive plant can grow as much as a foot or thirty cm per day in all directions, and chokes waterways as deep as twenty-five feet or six metres. With hydrilla, walking on water no longer requires a miracle. Too bad it makes lousy tea.

The German study can be found here. For more information on eDNA sampling by SLELO PRISM, visit their website. In Ontario, click here.

Photo of tea plant courtesy Wikimedia user James Steakley.