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I think any critter that travels in and out of flowers has the potential to be a pollinator. That might not be all it is though.
Yeah I agree with Dom the Nitidulid beetles is a natural pollinator of most Annona's. I've seen them on Sugar apple, custard apple, atemoyas, etc. In fact you can actually get some attractants for Nitidulid beetles to help get them to naturally pollinate. Hand pollination will always get the best results but natural help is welcome.