Honeybees are social insects that depend on their community for survival, making the health of new generations crucial. It’s no surprise that these bees devote considerable care and resources to their young. Nurse bees create a nutritious food blend from nectar and pollen to feed the larvae, using secretions from a gland in their heads. However, pollen frequently contains traces of various insecticides and other pesticides, exposing bee larvae to a complicated mix of chemicals.
Researchers from the Biocenter of Julius-Maximilians-Universität (JMU) Würzburg in Germany explored how EU-approved insecticides affect honeybee development, both alone and in combination with fungicides, particularly at concentrations typically found in the environment. Their findings were published in the scientific journal Environmental Pollution.
Previous studies have rarely looked into the effects of pesticide combinations at low doses relevant to the environment. “Our study highlights the urgent need to investigate this, as these substances can interact and amplify each other’s effects,” states PhD student Sarah Manzer, the study’s lead author. It’s also possible that some interactions occur only at lower concentrations, while others may be present at higher levels. “There are significant gaps in knowledge that we’ve started to fill with this research,” adds the JMU researcher.
The Experiments: Feeding the Bee Larvae
In their research, the scientists raised honeybees in a controlled lab environment and introduced various pesticides into their food, at natural environmental levels and at tenfold higher doses.
The JMU team fed the larvae the last neonicotinoid still permitted in the EU, acetamiprid, an insecticide that targets the oilseed rape beetle and other sucking pests. Previous neonicotinoids have been banned for being harmful to bees. They also administered a mixture of the fungicides boscalid and dimoxystrobin, along with combinations of the neonicotinoid and the two fungicides.
The higher concentration of the neonicotinoid resulted in a noticeable increase in larval mortality: 90.4% of the control group survived, compared to only 79.8% in the group exposed to the neonicotinoid. Manzer discovered that adult bees that consumed the neonicotinoid during their larval stage died significantly earlier than those in the control group, living on average 26 days instead of 31. However, at the environmentally relevant concentration, the neonicotinoid did not affect survival rates.
When larvae were fed solely the two fungicides, there was no notable effect on insect mortality; however, the hatched bees were lighter than those in the control group. Further studies are needed to determine the significance of this difference for their ongoing development and behavior.
Neonicotinoid Shows Complex Mixed Toxicity with Fungicides
The researchers were taken aback by the results when larvae were fed mixtures of pesticides: the lower dose of the neonicotinoid combined with the fungicides led to increased adult bee mortality, with the average lifespan being 27 days versus 31 days in the control group. Thus, a dosage of neonicotinoid that is usually considered harmless becomes dangerous when paired with fungicides. “This is concerning, as honeybees are exposed to numerous pesticides due to their wide foraging range,” notes Sarah Manzer.
Another unexpected result appeared with the mixture: the higher dose of the neonicotinoid, which harmed the larvae on its own, showed no impact on bee mortality when mixed with the fungicides.
Solitary Wild Bees May be More Affected
The combined effects noted by the researchers could potentially affect entire bee colonies, harming their subsequent generations. Additionally, solitary wild bees may be more vulnerable due to their direct exposure to pesticides, while honeybees in larger colonies might somewhat mitigate pesticide impacts.
The Würzburg scientists emphasize the need for further testing to deepen understanding of pesticide mixture effects.
