The situation of pollinators is becoming more apparent than ever. Scientists are increasingly documenting the decline in bees, butterflies and other pollinators. This is evidence that the buzzing of insects on many landscapes has become a whisper.

The threats to bees include habitat loss, climate change and intensive agriculture. Wild pollinators are losing their food sources as more of the flowering plants in fields become row crops and roads. Insects that forage in farms are often exposed to chemicals and suffer poor nutrition because they have not been provided with diverse food sources. Honey bees are a non-native, managed species that is imported into the US to pollinate crops. However, they still face dangers from poor nutrition and pests as well as pathogens.

New research in Nature has shown that certain threats kill more bees when combined than any one of them. The combination of several agricultural chemicals can have a synergistic effect, increasing bee mortality. This means that more bees die from the combination of chemicals than if they were all combined.

The paper’s authors analyzed 90 studies, which in all documented 356 bee-related effects, including chemical, nutritional, and parasite combinations. Each study had at least two factors that were harmful to bees. The stressors were categorized as if they could be combined, negated, or added together to cause additional damage. Compounding would indicate a synergistic result. If a pesticide caused 10% of bees death and another pesticide caused 15%, then the combined effect would be synergistic.

The researchers found repeatedly that bees exposed to multiple agrichemicals had a synergistic effect upon mortality. Combinations of stressors like nutrition and parasites had effects that were just as strong.

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It is still not clear why pesticides could have such an effect. The analysis showed that the bee stressors had no synergistic effect on non-lethal health measures like colony growth rates. Scientists have also found that pesticides can make bees more vulnerable to pathogens and chemicals. Elizabeth Nicholls from the University of Sussex, who is an ecologist who studies bees, said that compounding effects can also be caused by other processes. It could also be that their detoxification pathways are impaired if they are being bombarded repeatedly with chemicals.