When the bumblebees are suddenly silent

New study by KIT shows that pesticides reduce the buzz of bumblebees and thus the amount of pollen collected

What are the interactions between plants and insect pollinators? For the preservation of food security worldwide this question is of great political and economic importance. Even today, in some regions of China, pollinating fruit trees requires people to do the work of pollination by hand, simply because there are hardly any insects left to do so. But even in Germany many crops rely on pollination by insects. Crop protection products containing so-called neonicotinoids have been suspected for a long time to be responsible for the worldwide decline of beneficial insects such as bees and bumblebees. Neonicotinoids have become the most widely used insecticides in the world because they are extremely effective and convenient to use. For the first time researchers from the Karlsruhe Institute of Technology (KIT) and the University of Stirling in Scotland have discovered that pesticides significantly affect the ability of bumblebees to buzz pollination. 

Bumblebees are increasingly used as professional pollinators

Bumblebees are extremely efficient pollen collectors. They fly at very low temperatures well below 10 degrees Celsius to go in search of food. Per minute they can visit twice as many flowers as honey bees, they can transport more than these and come into contact with more pollen due to their size. Their famous humming comes about because, as they collect food with their muscle vibrations, they create frequencies that shake the pollen out of the flower. The bumblebees bite on a flower. The vibration shakes the pollen freely. One part they keep as food, the other part fertilizes other flowers. This process is called in the jargon also vibration pollination. More and more farmers are resorting to bumblebees because of these diverse characteristics. In the Netherlands, bumblebees have been used extensively in the breeding of tomatoes and peppers since the mid to late 1980s. But also in strawberry cultivation or on apple orchards bumblebees are extremely useful and can be found on many plantations in Europe as professional pollinators.

Learning ability of bumblebees restricted

Dr. Penelope Whitehorn, biologist at the Institute for Meteorology and Climate Research - Atmospheric Environmental Research (IMK-IFU) of Karlsruhe Institute of Technology together with her colleague Dr. med. Mario Vallejo-Marin, University of Stirling examined the effect of a neonicotinoid pesticide on bumblebees. Their research shows that it has a negative influence on the vibrations during pollen collection, and therefore also the buzzing. For this purpose bumblebee colonies were monitored, which were burdened by the pesticide. The researchers recorded the buzzing over a longer period by microphone. Afterwards, they analyzed the acoustic signal that the bumblebees produce during vibration pollination to detect changes in the buzz.

In the course of the investigation, they found that the pesticide exposure reduced the vibrations. As a result, the amount of collected pollen decreased and with it the amount of food for the bumblebees. Bumblebees from a control group that were not exposed to the pesticide gradually learned to develop more pollen and pollinate flowers. "The bumblebees that came in contact with the pesticide did not develop their skills," says Penelope Whitehorn. At the end of the experiment, they collected between 47 and 56 percent less pollen than the control group.

Now the researchers want to find out what exactly happens with the bumblebees. One hypothesis seems obvious: The aggressive pesticides affect the memory and cognitive abilities of bumblebees. Both, however, are important prerequisites for the complex behaviors of bumblebees. Studies have already shown in bees that even at lower doses the navigation system and learning are negatively influenced and paralysis symptoms occur at high doses. The likelihood of similar effects in bumblebees is relatively high.

Text: Oliver Jorzik (ESKP), final check and scientific advice Dr. Penelope Whitehorn (Karlsruhe Institute for Technology, KIT)

  Whitehorn, P. R., Wallace, C. und M. Vallejo-Marin (2017): Neonicotinoid pesticide limits improvement in buzz pollination by bumblebees. Scientific Reports Volume 7

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