An estimated 15,000 honey bees were killed after an error by council workers.
Anglesey council's pest control was called to deal with the insects near Cemaes on Friday but said its "usual procedure was not followed".
If honey bees are present on a callout, a beekeeper is usually asked to help.
Katie Hayward of Felin Honeybees said she was "furious" and thought about 15,000 bees had died. An investigation has begun into the incident.
Bees are a protected species and in most cases, it is illegal for people to kill them.
A council spokeswoman was unable to give an estimate on the number of dead bees.
Ms Hayward said: "It's such a shame because we have been working hard for two years to highlight why these bees are so important. All that work has been tarnished by this one incident.
"Had the council contacted me it would have taken me two minutes to save the bees."
The council's own website says the most appropriate method of control for bees is "the safe removal of a hive".