If biologists want to know how the world of insects is doing, then they don’t necessarily have to use net traps or go outdoors: a tea bag or a sample of dried herbs can also reveal a lot about the number and species spectrum of insects. This is possible because the animals leave traces of their DNA on and in the plants they colonize, feed on or lay their eggs on. These genetic traces can then be identified using DNA analysis, researchers have found.
The great diversity of insect species is of immense importance for our environment because insects pollinate countless plant species and are an important source of food for many animal species. It is all the more important to keep an eye on their dynamics and interactions with the environment – especially in view of the increasing insect mortality caused, among other things, by the increased use of pesticides. So far, biomonitoring has mainly been carried out by randomly catching insects. However, this method reveals little about the interaction between plants and insects and usually ends fatally for the animals already affected by the population decline.
A more modern approach takes advantage of the fact that insects leave tracks everywhere on plants – be it through bite marks, eggs or faeces. These always also contain the genetic material of the animals. Scientists can therefore use this environmental DNA (eDNA) to draw conclusions about which insects the plant was in contact with. However, the surface of plants is a rather unsafe place for the valuable eDNA, as it can be destroyed by UV light or washed away by rain.
Insect DNA in tea
For this reason, Henrik Krehenwinkel from the University of Trier and his team have developed an innovative technique in which the eDNA is not taken from the surfaces of the plants, as is usually the case, but from crushed, dried plant material. “This offers a particular advantage: when dried, DNA is stable and suitable for long-term storage,” explain Krehenwinkel and his colleagues. To test their method, the scientists use dried plant material found in every household: “Tea and herbs are made from a variety of plants that are grown all over the world and can therefore tell a lot about specific insect-plant- interactions and even about the location,” the researchers explain.
With the new method, they actually succeeded in detecting the DNA of a total of 1279 insect species in the dried plant samples. “We found the DNA of up to 400 different insect species in a single tea bag,” reports Krehenwinkel. Green tea contained the greatest diversity of eDNA from different insect species. The new biomonitoring method also revealed something about the origin of the insects. In peppermint tea, the scientists found traces of insects that are mainly found in northwestern North America, a major growing region for peppermint, while eDNA from typical East Asian species was only found in green tea.
look in the past
A great added value of the new method is the access to stable DNA, which has been in the plant material for a long time and thus allows a glimpse into the past. Krehenwinkel and his colleagues therefore want to use plant collections that have been archived for decades to test whether environmental DNA can also be used to trace the development of insect populations over a long period of time. In this way, important conclusions could be drawn about the causes of insect mortality or, for example, the spread of plant pests could be researched. In addition, the new method also extends the number of observable species: “Now we can also prove which insects live inside the plant,” says Henrik Krehenwinkel, describing the new horizon of knowledge.
Finally, the procedure could also become a case for criminalistics. Reliable statements about the actual geographic origin of plants can be made using eDNA. In this way, customs could determine whether imported types of tea actually come from the specified countries. And what is possible with tea also applies to other plants, such as cannabis.
Source: University of Trier; Specialist article: Biology Letters, doi: 10.1098/rsbl.2022.0091