The special effects for the bee attack sequences were quite simple. Often footage of swarming bees would be superimposed over footage of the actors and fake plastic bees would be glued to the actors. Some shots of swarming "bees" was actually footage of coffee grounds, floating and swirling in water tanks, that was superimposed over landscape footage.
Watch for the guitarist for the group The Birds at the beginning of the movie - it's a pre-Faces, pre-The Rolling Stones Ronnie Wood. He's on the far right of the screen.
Freddie Francis years later admitted that he should not have had Robert Bloch's script rewritten, as it only made things worse.
The exteriors of the Hargrove farm were built entirely on sound stages, complete with fake trees, fences and sheds.