They are usually not visible from the outside, but are numerous: A hidden variety of insects live in the stalks of perennial grasses – from gall midges, beetles and small flies to wasps and wasps that parasitize these stalk inhabitants. This is what biologists discovered when they examined the inner workings of more than 23,000 blades of grass. However, their results also show that this diversity is only found in perennial grasses. If meadows are mowed frequently, the insects lack these refuges.
Insects have a hard time these days: Fields are often monocultured, making it difficult for them to find food and living space. Pesticides affect their reproduction, orientation and survival, and the over-fertilization and intensive management of many pastures and grasslands also limits their life chances and diversity. As a result, the number of insects has declined in Germany and many other countries. However, many studies of insect biodiversity focus on flying species or other species that are easily studied using traps.
Searching for insects inside blades of grass
But there are many insects, especially in grasslands, that are left out: “Grasses are well-known host plants for external insects that feed on them, such as grasshoppers or bugs. But the endophagous insects that live in the roots or produce galls are often hardly taken into account,” explain Teja Tscharntke from the University of Göttingen and his colleagues. In order to change this, the biologists examined the inner workings of more than 23,000 stalks of common grass species.
The grass species sampled included five annual grass species such as blackgrass and windstalk, as well as ten perennial species such as taffeta grass and couch grass. The team collected the grass blades in autumn and winter and dissected the insects and insect larvae living in the blades. They continued to breed the larvae in the laboratory so that they could then determine the species more precisely based on the adult animals. They then determined the position in the food web of the identified insects.
Amazing diversity – but only in perennial grasses
The inventory revealed an astonishing diversity of grass inhabitants: “In the ten perennial grass species, we found a total of 83 herbivorous species and 172 parasitoid insect species,” report Tscharntke and his colleagues. Common herbivores included gall midges and stalk flies, which overwinter mainly as larvae in the stalks. The parasitoids were mostly wasps such as the orchid wasps, which lay their eggs in the herbivorous insects and ultimately kill them. On average, the biologists counted almost twelve herbivorous insect species and 30 species of parasitic wasps in each perennial grass species. The longer the stalks were, the greater the variety of insects that lived on them.
What was astonishing, however, was that Tscharntke and his team did not find a single insect in the stalks of the five annual grass species. This was the case even when the annual grasses had stalks of similar length to the perennials. “This strict dichotomy between annual and perennial grasses was surprising,” the team writes. You see several possible reasons for this. On the one hand, perennial grasses offer a more reliable resource because they can be found in the same place over a longer period of time. In addition, the perennial grasses are larger on average and have thicker stalks. This offers larger insect species in particular more food and space for the larvae.
Unmowed areas are important
According to Tscharntke and his colleagues, these results underline how important it is to leave as few parts of the pastures and meadows unmowed for long periods of time. Only then will the stalk-dwelling insects find the refuges they need for their survival. “The hidden wealth of insect species in blades of grass is unfortunately largely ignored by grassland management, even though most species depend on the undisturbed development of grasses,” says Tscharntke. Regular mowing threatens the diversity of the many highly specialized insects in the grass.
Source: Georg August University of Göttingen; Specialist article: Basic and Applied Ecology, doi: 10.1016/j.baae.2026.01.004