Why are the heads falling off my dahlia?

[Katisha]

The whole flower heads just seem to fall off, right where the stem joins on, as if its been very accurately cut off. I cant see any earwigs and its not just the petals - its the complete flower head just dropping off ij its prime. I cant find any reference to this online. Does anyone know?

Are you quite sure that nobody is cutting them off on purpose?

Wasps?

I've seen more on my dahlias this year and I think they like munching the stems? Especially the juicy newer stems, which tend to be where the flowers are.

@Merapi yes - one fell off in front of my eyes! Like a clean cut at the base of the flower head.

I'd say wasps too, I've read something about them eating dahlia stems. Otherwise, are you sure it's not slugs and snails? I've lost all the buds from one dahlia as they've been munched off - evidence of slime tells me the culprit.