Hidden insect diversity in grass shoots threatened by mowing

Two female parasitoid wasps depositing their eggs in the larvae of the gall midge hidden in a reed shoot. The tiny gall midge larvae feed within plant tissue creating bumps, known as “plant galls”. The wasp eggs will hatch and their offspring will feast on the baby gall midges.
Photo Credit: Tscharntke, T. et al., Basic and Applied Ecology
(CC BY 4.0)

Scientific Frontline: Extended "At a Glance" Summary

The Core Concept: A research initiative revealing a diverse, overlooked ecosystem of 255 insect species inhabiting the shoots of perennial grasses, demonstrating a complex web of herbivores and parasitoids often ignored by conventional biodiversity studies.

Key Distinction/Mechanism: Unlike annual grasses, which were found to harbor no specialized stem-boring insects, perennial grasses support intricate food webs where shoot length correlates directly with species diversity. The stability of these perennial shoots allows specialized larvae to develop and overwinter, a cycle disrupted by mechanical interference such as mowing.

Major Frameworks/Components:

• Species Categorization: Identification of 83 plant-eating species (e.g., grass flies, gall midges) and 172 natural enemies (parasitic wasps).
• Trophic Analysis: Mapping of the food chain from host grass to herbivore to parasitoid predator.
• Habitat Comparison: Contrast between ten perennial (long-lasting) grass species and five annual (short-lived) species, searching over 23,000 shoots.
• Specialization Metrics: Finding that nearly two-thirds of the insects are specialized to grasses, with half restricted to specific grass species.

Branch of Science: Agricultural Ecology, Entomology, and Functional Agrobiodiversity.

Future Application: Development of new grassland management protocols that incorporate unmown "refuge" areas to sustain insect populations and enhance natural pest control via parasitoid wasp conservation.

Why It Matters: This research exposes a critical gap in current conservation strategies, showing that standard grassland management (regular mowing) unwittingly destroys the habitats of specialized insects essential for ecological stability and biodiversity.

The number of insect species (plant-eating as well as parasitic wasps) depending on the length of the grass shoots. The small empty squares on the diagonal red line mark the average shoot length and the total species diversity of the ten perennial grass species, which are also shown in the illustrations under the line. The black squares on the horizontal line indicate the values for the annual, five short-lived grass species.   
Photo Credit: Tscharntke, T. et al., Basic and Applied Ecology
(CC BY 4.0)

When it comes to biodiversity, researchers and the public tend to focus on large-scale patterns. This overlooks a hidden but precious diversity: small, inconspicuous wasps, midges, flies, beetles and other insects that live in plants. These tiny creatures are actually very common, as shown by a team of researchers at the University of Göttingen and the Hungarian HUN-REN Centre for Ecological Research. The researchers measured, dissected, and searched for insects in over 23,000 shoots of grass. They found 255 species of insects in ten perennial grass species, which last year-round, but not even one in five annual, short-lived grass species. The longer the shoots of the perennial grass species, the higher the diversity of insects found in them. Around a third of the insect species feed directly on the grass. The remaining species, mostly wasps, live parasitically on the insects that feed on plants. Almost two-thirds of insects specialize in grasses, half of them even in specific grass species. The conclusion is that areas in grassland should not be mown for several years: stable insect populations need undisturbed refuges with intact shoots of grass. The results were published in Basic and Applied Ecology. 

The team studied ubiquitous grasses that occur in large populations in many regions. These included five annual species such as black-grass and wind-grass, and ten longer-lasting species such as cat grass and couch grass. In autumn and winter, the researchers collected all insects from the shoots and classified them according to their respective species. They raised the larvae in the laboratory so that they could be clearly identified. They then analyzed the “food web” – the multiple food chains – between the grasses, the plant-eating insects and the parasitic wasps that are their predators. 

This revealed the diversity of insects hidden in shoots of grass. Eighty-three of the species found are plant-eaters such as grass flies and gall midges. The remaining 172 species are their natural enemies: tiny parasitic wasps whose larvae develop on or inside a host and attack and ultimately often kill it. On average, each perennial grass species is home to 12 plant-eating insect species that are attacked by 30 species of parasitic wasps. According to the data, perennial grass species with longer shoots attract more insects. This is explained by the fact that they are more visible and productive host plants with a more diverse food supply. The difficulty in predicting when and where short-lived annual grasses will grow seems to be the reason why only a few insects have evolved to specialize in them. 

Professor Teja Tscharntke
Photo Credit: T Tscharntke

Regular mowing threatens the diversity of these many highly specialized insects. “The hidden wealth of insect species in shoots of grass is unfortunately largely ignored by grassland management, even though most species depend on the undisturbed development of grasses,” says lead author Professor Teja Tscharntke at Göttingen University. This community depends on intact grass shoots overwintering in minimally disturbed habitats. Therefore, unmown long-term refuges are needed. “The management of grasslands should pay much more attention to the neglected community of specialized insects in grass shoots,” adds Tscharntke. 

Published in journal: Basic and Applied Ecology

Title: The hidden multitrophic diversity of specialized grass-shoot insects – neglected by grassland management

Authors: Teja Tscharntke, Péter Batáry, and Stefan Vidal

Source/Credit: Georg-August-Universität Göttingen

Reference Number: agro021426_01

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