Two vandals branded "braindead morons" have been captured on CCTV attacking the hives of endangered bees.
Conservationists believe as many as 10,000 rare bees may have died in the attack at Wisbech Castle, Cambridgeshire.
The hives were established in the castle grounds last July in a bid to boost numbers of British black bees.
A police spokeswoman called the attack a "cruel, unprovoked and completely unnecessary act of violence."
CCTV footage, captured at about 05:15 GMT on 8 January, showed two people breaking into the castle grounds and lifting the lids off the hives, before kicking them and attacking the bees inside with sticks, police said.
Steve Tierney, a councillor who chairs the Wisbech Castle Project, said on Facebook that the castle had been targeted by vandals before, but that "ridiculous and counterproductive heritage rules" prevented stronger security measures, such as wall spikes and anti-climb paint.
Describing the culprits as "braindead morons", he said: "We had to make sure the bees were settled, and they've been doing well - but importantly they cannot be disturbed over the winter months.
"These two people scaled the wall and went through black bin bags before smashing the lid off one of the hives.
"They ran off and returned, setting about smashing them up. They may have been stung and came back for revenge."
PC Kirsty Hulley, of Cambridgeshire Police, said castle staff were "devastated" by the attack.
"They estimate as many as 10,000 rare bees have died, but are unable to check properly until March as further exposure to the cold air would kill the remaining bees.
"It will cost the museum around £2,000 to restart the project in the spring."