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BIODIVERSITY

Hunting for insects on farmland

Photography: René Manger

KENNISONLINE 2020


Insects are struggling in the Netherlands, especially in rural areas. To get a better understanding of the effect that measures have on the restoration of biodiversity, Wageningen scientists set insect traps on grassland on five farms.

Mosquitoes and bluebottles are not exactly popular, and fleas, horseflies and wasps even less so. Yet many people are deeply concerned about the global decline in insect populations, as insects are indispensable for pollinating our fruit trees and crops, while also being a food source for birds and bats. The many different species of insect also keep one another in equilibrium: ichneumon wasps are parasites of caterpillars, for example, and ladybirds eat aphids.

A healthy ecosystem is brimming with insects, both above ground and in the soil. That ecosystem is currently faltering. The Dutch researcher Caspar Hallman caused a huge stir in 2017 when he reported that insect populations in 63 German nature areas had fallen by 76 per cent since 1989. Ecologist Anne Schmidt of Wageningen University & Research suspects the decline in the Netherlands will have been even steeper, especially in agricultural areas.

Very little data

There is very little data available on the decline in insect populations in agricultural areas, but according to Schmidt, “increasingly intensive farming practices are probably a key reason for the loss of biodiversity in rural areas. Hence the call for a different kind of agriculture that is more nature-inclusive.”

Flies, spiders, beetles. Ecologists Dianne Sanders and Inge de la Riviere of Wageningen University & Research examine how many small insects can be found on farmland, in order to measure the state of biodiversity. They use a special insect trap for this purpose.

Schmidt and her colleagues at Wageningen Environmental Research are currently working on the scientific evidence underlying the guidelines on nature-inclusive agriculture, with the aim of developing effective, nature-inclusive business models for farmers. These business models are being developed in the Dairy Farming Biodiversity Monitor and the Biodiversity Restoration Plan. The idea is to reward farmers for taking measures that restore biodiversity. The challenge now is to find instruments that can unambiguously determine whether biodiversity has in fact improved.

Thirty-year-old grassland

Via the association of farming collectives BoerenNatuur, the Wageningen researchers came into contact with the Midden Overijssel farming collective. The Wageningen researchers will now be studying five farms in Staphorst over four years to compare the biodiversity of grasslands ranging in age from brand-new to 30 years old. “We want to get a better idea of the effect of grassland regeneration on the biodiversity of insects,” explains Titia Wolterbeek, director of the Dutch butterfly society Vlinderstichting. “We’re expecting biodiversity to become richer, both in terms of numbers of species and in terms of biomass, as the grassland ages.”

The many different species of insect keep one another in equilibrium. For example, ladybirds eat greenfly. Photo: Shutterstock

The European Union pays grants worth billions of euros to farmers for restoring nature in what is termed ‘permanent grassland’. The idea is that a richer soil life will develop with more biodiversity if farmers are not constantly ploughing the field, sowing grass again or turning it into a maize field, for example. The criterion for ‘permanent grassland’ is that it must be left untouched for six years. “But why six years?” wonders Anne Schmidt. “There’s little or no scientific basis for choosing that particular limit. We want to see what it means for biodiversity if you leave the grassland untouched for longer and manage it using natural methods, using farmyard manure rather than artificial fertilizer — what effect does that have on insects?”

Equilibrium

Fields that are constantly being ploughed up and heavily fertilized may still have a lot of insect life, but it will be restricted to just a few common species. The researchers are expecting to find more unusual and specialized species in older grassland. “Healthy soil is teeming with life,” says Titia Wolterbeek of the butterfly society. “It is a rich ecosystem with many different species such as moths and mosquitoes, as well as worms, spiders, centipedes and woodlice. They live in each other’s passageways and digest organic material. They use one another as food or live in a parasitic relationship. As long as there are enough different species, the system remains in equilibrium and the soil stays healthy. It also helps to have sufficient numbers of the right host plants — the plants where caterpillars live or butterflies lay eggs on.”

Various species of moths are attracted to the special trap, a bucket with LED lights. Image: René Manger

In grassland with little variation, there is not much available for many species of insects. That is why the field research will also encompass the borders of the grassland. Many butterflies can survive in the verges if they are managed in a nature-friendly manner. The caterpillars or larvae of many insect species live underground. They live off what they can find there. If all they find is perennial ryegrass, that is pretty limited. Other insects live above ground but they pupate underground and the pupa then lives underground for a while.

Nationwide measurement network

The butterfly society has had a nationwide measurement network for butterflies ever since 1990. A similar network was added for dragonflies in 1998 and one for moths in 2012. “We developed a special trap to monitor the moths, a bucket with a fitted LED light that runs on a battery,” explains Titia Wolterbeek. “The light turns on automatically when the sun sets and turns back off when the sun rises. Previously most of our measurements had been done in gardens but you can use the new trap anywhere, including in places without an electricity supply such as nature areas, arable fields or pastureland.” The moths caught in the trap hide in egg boxes. Early the next morning they are counted, identified, perhaps photographed, and then released.

A pyramid trap in grassland for catching insects. Photo: Dianne Sanders

Fieldworker Rene Manger explains how a LED bucket works. Photo: Anne Schmidt

Moths are fairly easy to identify but flies, mosquitoes, hoverflies and bees require expert knowledge. They are among the biggest groups of species in the Netherlands. That is why the Wageningen researchers are using ‘pyramid traps’ in addition to the buckets. These traps, which are pyramidal in shape, are placed in the grassland. At the top is a small pot in which the insects living in the grassland or the trees are trapped. They are then identified by experts back in the lab. What matters is not just the diversity of species (the various families of moths, beetles and mosquitoes) but also the overall biomass of insects, because that determines the food supply for insectivores such as birds and bats.

Farmers

Schmidt: “The farmers involved are very curious to find out what insect species live on their land. Our top priority is to answer scientific questions but the great thing about this project is the close involvement of the farmers. Science often operates in an ivory tower. We want to do more with people on the front line.”

The researchers hope the information from the traps on the five farms will give a clearer picture of the effect that grassland rejuvenation has on biodiversity. Titia Wolterbeek: “We hope this will give us a better understanding of the right measures for restoring biodiversity in farmland.”

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