Scientists at Royal Holloway University of London said they found the result in bumblebee colonies with prolonged exposure to a pyrethroid pesticide, used on flowering crops to prevent insect damage.
They tracked how the bee colonies grew over a 4-month period, recording the size and weight of bees as well as monitoring the number of queens and male bees produced by the colony.
"We already know that larger bumblebees are more effective at foraging. Our result, revealing that this pesticide causes bees to hatch out at a smaller size, is of concern as the size of workers produced in the field is likely to be a key component of colony success, with smaller bees being less efficient at collecting nectar and pollen from flowers," researcher Gemma Baron said.
With an existing European Union on the use of three neonicotinoid pesticides, the use of other classes of pesticides such as pyrethroids is likely to increase, the researchers said, with an impact on bee colonies.
"Bumblebees are essential to our food chain so it's critical we understand how wild bees might be impacted by the chemicals we are putting into the environment," researcher Mark Brown said. "We know we have to protect plants from insect damage but we need to find a balance and ensure we are not harming our bees in the process."