Bees exposed to just one dose of pesticide have fewer offspring and can take generations to recover, a study has found.
Researchers analysed the effects of imidacloprid, a neonicotinoid pesticide, on bees.
Pesticides have been linked to population decline in bees, but researchers have never studied the generational effects of the chemicals.
Researchers Clara Stuligross and Neal M Williams, of the University of California, exposed bees to the chemical in a controlled setting.
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They found that bee larvae exposed to the chemical produced 20% fewer offspring after they became adults.
They also found that multiple exposures led to even fewer offspring.
Those that were exposed as larvae, for example, and were then exposed again as adults had 44% fewer offspring, which, the researchers noted, represented approximately 10 fewer offspring out of a normal group of 24.
They captured a large number of blue orchard bees, a type that is similar to the honeybee but has different colouring and collects pollen on its belly instead of its legs.
These bees were separated into small groups, each of which was exposed to imidacloprid in different ways.
The findings showed that the use of imidacloprid on agricultural crops – which is banned in Europe but not in the US – can lead to dramatic reductions in bee populations due to reductions in fertility rates.
Greenpeace has warned that loopholes in EU law have allowed some farmers to continue using neonicotinoids after the ban.
The findings are important to farmers who grow tree-bearing crops, such as almonds, peaches, apples and cherries.
The researchers wrote: "We reveal that pesticide exposure, both directly to foraging bees and via carryover effects from past exposure, dramatically reduced bee reproduction, which reduced population growth.
"Carryover effects reduced bee reproduction by 20% beyond current impacts on foraging bees, exacerbating the negative impact on population growth rates.
"This indicates that bees may require multiple generations to recover from a single pesticide exposure; thus, carryover effects must be considered in risk assessment and conservation management."
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