Nothing you can do, or nature can do... almost without exception citrus flower need no help to produce fruit; bees help if you want seeds; but that is another dirty little secret that Argentina knows too well

The only reason to hand pollinate a citrus flower is when attempting to produce new hybrids.

'Nothing you can do or nature can do..'

The effect of external application of gibberellin and auxin hormones on citrus fruit set is the subject of ongoing scientific investigation and trials.. I would not be surprised if there's already some pilot commercial application.