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Where have all the bees gone?

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​​​​Insects are essential for many plants in our fields and plantations. Alongside the honey bee, a whole armada of wild insects makes sure that flowers are pollinated and produce fruit. However, the colonies under the care of beekeepers as well as their wild relatives are falling in numbers. Fewer and fewer bumblebees, bees and hoverflies are humming in the fruit trees and buzzing across the fields. While looking for the causes, it was discovered that no blanket solutions exist, and that it is worth taking a closer look at the ecosystems of arable lands and their surroundings.

235 to 577 billion US dollars: That is the value of the service that pollinators such as honey bees, wild bees, butterflies, bats and birds provide worldwide every year. They transport the pollen from one flower to another and they do it free of charge. There's just one catch: the number of flower pollinators has declined rapidly in recent years. Many species are endangered, threatened with extinction or already irretrievably lost. This is not only a cause of concern for environmental protection and science. Some of our crops produce either considerably less or no fruit without these flower visitors. If the pollinators die, food security is also at risk. 

The urgency of the problem is already evident today, in regions where there are hardly any wild flower visitors: in some regions of Asia, people pollinate the flowers in the orchards with brushes. In Japan and the USA, mini drones are developed to do the job instead. How can research help to save the pollinators before it is too late? 

A team from the Leibniz Centre for Agricultural Landscape Research (ZALF) and the Johann Heinrich von Thünen Institute have embarked on a search for answers. "There are many different causes", Karoline Brandt explains. "The interrelationships between the organisms, their environment, the areas under cultivation and the type of cultivation are complex." But the list of the threats is long. Whether monocultures, destroyed natural habitats or pesticides – in intensified agriculture, insects find less and less food and fewer basic living conditions. Their populations are weakened, making them more susceptible to disease and parasites. Brandt says, however, that it is difficult to precisely quantify individual causes or make precise statements about their interactions. Up to now, individual influencing factors have been examined separately from each other – with partly contradictory research results.


With attention to detail

To remedy this situation, the researcher and her team spent three years investigating fields in Mecklenburg-Western Pomerania, Thuringia and Bavaria. They questioned how well suited these are as habitats for Bumblebees and Co. They selected three farms, two conventional and one organic, with 15 fields each for their survey . First of all, they had to determine which pollinator species occur on the fields and in what numbers. 

From April to October, insect traps were set up and inspected every two weeks. In total Brandt and her team identified 157 species. The 23 most common, which belonged to six different groups of insects, were analysed in more detail. At the same time, they recorded which crops were growing at a given time, how high the plants were, how much soil they covered and which wild herbs were present. In addition, they also took a closer look at the surrounding area within a radius of one kilometer: Were the neighbouring areas also farmed? Were there any settlements nearby? How high was the proportion of forest, grassland or semi-natural shrubbery, hedges and fallow land? The results show which living conditions have a positive effect on these pollinating insects and the scope of action available to agriculture to support them.


Different bees, different requirements

So far, the results show above all one thing: hoverflies, honey bees, bumblebees, but also the lesser-known sweat bees, mining bees or mason bees – they all are frequent and important guests in the areas observed. However, their demands on food sources and nesting possibilities vary greatly. Whereas mining bees require a dry place with loose soil for example, some types of bumblebees require cavities close to the ground for their nests. Many species of mining bees are only active as flying insects in spring and accordingly need food in the form of pollen and nectar. In the case of the hoverfly, it is the other way round, as it does not start its high season until August. Bumblebee colonies, on the other hand, are on the wing from spring to autumn – and correspondingly hungry. "We need to take the differences in lifestyles between the individual groups into account. It cannot be said in general terms that this or that measure is good for flower visitors", explains Brandt, summarising the results. Before implementing conservation measures in an area, it is important to find out which pollinating insects are found there under natural conditions in order to specifically promote these. "There is no single solution for everything."

The challenge is to protect as many different species of pollinators as possible, taking into account their individual needs, because this is the only way to ensure that the required pollination performance is sustained. For example, the role of wild bees has so far been underestimated. Recent studies show: These highly efficient pollinators, do not usually live in colonies but solitary, pollinate crops twice as effectively as honey bees, also fly at low temperatures and visit more flowers. 

Particularly for those insects that still require an abundant supply of food in late summer and autumn, survival in agricultural landscapes is usually difficult. "The problem is often the so-called food gaps", Brandt explains. Rapeseed monocultures, for example, provide nectar and pollen in abundance for three to five weeks in early summer. Once the rapeseed has flowered, however, the pollinating insects are facing lean times. If there is a beekeeper on site, he or she will take the bee colonies to more prolific feeding grounds. However, this shuttle service is not available to wild insects. Only wild herbs blooming on field margins or other flowering crop plants save them from starvation. 

Extended crop rotations, which provide a variety of food over a long period of time, offer alternatives. Instead of relying on the same crop plants such as maize or wheat, for example, clover, legumes or new energy crops like the cup plant, could be integrated into the crop rotation as an attractive additional source of nectar and pollen. Flower strips of wild herbs that bloom throughout the year can also close the food gap in critical periods. 

A further reason for the long-standing dramatic decline in insect populations is not only the lack of basic food resources, but also a frequently used group of insecticides: Neonicotinoids, which are highly effective and attack the​ insects' nerve cells, are absorbed by the plants and degrade very slowly. The crop plants are indeed protected against pests, but the active ingredient also finds its way into pollen and nectar. Laboratory studies have shown its harmful effects on pollinating insects. Under field conditions, the research results to date are not conclusive, as there is still too little data available.​


Existential for agriculture

An intact pollinator system is not only existential for consumers, but also for agriculture. "Many people are aware of how important the performance of bees and bumblebees is for their crop plants" Brandt says. "But they often shy away from the strict EU directives when it comes to concrete action."​ In order to obtain funds for the loss of yield, a flower strip on the edge of the field, which provides insects with food and habitat for their entire lifespan, must be sown at a precisely defined time and with a precisely specified width. If the farmer deviates from the guidelines, he can expect significant financial losses. And the bureaucratic requirements for farmers are often so high that they would rather continue as before. Brandt would like to see more flexibility and pragmatism from politicians in order to facilitate the transition to insect-friendly agriculture. 

Results show: there is no one right answer. Every environment has its own pollinator community that needs protection. Using the data obtained, the researchers are hoping to use ecological models to predict how the insect community will develop in the future, depending on the crop grown and the landscape features. One thing is certain for Brandt: "If we want to maintain the pollinators' performance, we need to p​reserve the biodiversity of the landscape as well as the crop species and understand their interrelationships." 


Text: Heike Kampe ​


Karoline Brandt
is a Geography graduate. After her studies at the Humboldt-University of Berlin, she started researching different groups of organisms in agricultural landscapes at the Institute for Land Use Systems at ZALF in 2011. Today, she works at the Thünen Institute for Biodiversity in Braunsc​hweig.


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