Bumblebees could be facing extinction as inbreeding in colonies turns hard-working female bees into useless males, scientists have found.
Male bumblebees do not help around the nest
The bee populations are now mainly confined to nature reserves - isolated by intensively farmed land with no other bees around.
This forces them to mate with relatives, the study found.
Male bees are "basically lazy", said study leader Dr David Goulson of the University of Southampton.
A bumblebee queen usually produces a large number of worker daughters to help in the nest and with gathering nectar and pollen.
But if the queen mates with a relative, many of the genetically female offspring develop into sterile males.
"Since male bumblebees do no work, and have only one purpose - mating - a sterile male is worse than useless," said Dr Goulson.
"If the queen is producing sterile sons instead of worker daughters, the nest
is probably doomed.
"This means that, even on well-protected nature reserves, the last populations of these rare insects may be driven to extinction."
Researchers studied a number of species, including the Moss Carder Bee (Bombus muscorum), at various sites across the UK, from the Hebrides in Scotland to Dungeness on the Kent coast.