New insecticides attack the intestinal flora of bees

honey bee

Bees are often exposed to dangerous insecticides when collecting pollen. © kojihirano/iStock

Many insecticides are now known to harm bees. It has now been shown that two toxins originally labeled as bee-friendly are even more dangerous for the animals than previously assumed. Accordingly, insecticides based on flupyradifuron and sulfoxaflor attack the intestinal flora of the bees, making them more susceptible to diseases and allowing them to die earlier. This effect is even more pronounced when the agents are combined with a common fungicide.

Insecticides are designed to protect crops from pests while sparing beneficial insects such as bees whenever possible. For a long time, the widespread agents from the neonicotinoid class were considered effective but largely bee-friendly. These insecticides damage the nervous system of herbivorous insects, leading to convulsions and eventual death. In the meantime, however, it has been shown that these substances also harm bees, they are considered to be partly responsible for the death of bees and some neonicotinoids are now banned in the EU.

Feed tests in the laboratory

As a replacement for the banned neonicotinoids, the substitutes flupyradifuron and sulfoxaflor, among others, were developed a few years ago, the effect of which is based on the same biochemical mechanism. It has now been shown that flupyradifuron and sulfoxaflor also harm bees. In the EU, sulfoxaflor has therefore only been allowed to be used in greenhouses since 2022. However, the exact extent of the bee harmfulness of these two substances is only gradually becoming apparent, currently in a study by a team led by Yahya Al Naggar from the Martin Luther University Halle-Wittenberg (MLU). To do this, the researchers bred honey bees in the laboratory and initially fed them sugar syrup. After a few days, they divided the animals into several groups and mixed a different pesticide into the food of each group. One received flupyradifuron, another sulfoxaflor.

In two other groups, azoxystrobin ended up in the food in addition to one of the toxins. Azoxystrobin is an agent against plant fungi that is also often used in agriculture in combination with insecticides. When it came to the dosage of the insecticides, the scientists oriented themselves “on realistic concentrations, such as those found in the pollen and nectar of plants that have been treated with the pesticides,” explains lead author Yahya Al Naggar, who was working at the MLU at the time of the study. A control group continued to receive only the normal sugar syrup. For ten days, the scientists observed what the different foods did to the bees.

Chaos in the digestive tract

The result: In the flupyradifuron group, about half of all bees died. Even more so in the group that also received azoxystrobin. Sulfoxaflor had a similar effect on the animals, but more of them survived. What is new is that the animals die, among other things, because toxins and antifungal agents damage their intestinal flora. “The fungicide azoxystrobin reduced the naturally occurring fungi very significantly. That was to be expected since fungicides are used to combat fungi,” says co-author Tesfaye Wubet from the Helmholtz Center for Environmental Research. But not only that: the agents also ensure that the bacterium Serratia marcescens can spread in the digestive tract of the animals. According to the researchers, this type of bacteria makes the animals more susceptible to diseases, which ultimately causes them to die prematurely.

Since the study was carried out under controlled laboratory conditions, it is unclear whether the bees' intestinal flora reacts in the same way to the various toxins in the wild. "It could be that the effects would be even more dramatic - or that the bees would manage to fully or at least partially compensate for the negative consequences," explains Wubet. In any case, the study demonstrates that even newer crop protection products are far from being bee-friendly. In Germany, the funds are only allowed in the greenhouse, but that says nothing about their worldwide use. The research team hopes that insecticides will be researched even more intensively in the future to determine whether and how they harm beneficial insects. According to them, the consequences for the intestinal flora should also be an integral part of such tests in the future.

Source: Martin Luther University Halle-Wittenberg (MLU) and Helmholtz Center for Environmental Research (UFZ) ; Specialist article: Science of the Total Environment, doi: 10.1016/j.scitotenv.2022.157941

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