If you’re worried about Nargles in your mistletoe then you’ve probably been reading too much Harry Potter, for that’s the only world where they occur. If indeed they occur at all.
Even in the Potter world the only evidence of their existence is from Luna Lovegood, a fellow Hogwarts student, who suggests they are mischievous beings who steal things. And often live in mistletoe.
Her mistletoe remarks are made in Chapter 21 of Harry Potter and the Order of the Phoenix. Harry is discovered alone in a roomful of Christmas decorations by Luna. She points out he is standing under some mistletoe. As he hastily retreats she suggests that he’s wise to do so as ‘it’s often infested with Nargles’. Harry has never heard of Nargles but doesn’t admit this.
Three pages later he’s in the same spot, but this time alone with Cho Chang, a young lady who has made it clear she, er, likes him. She, like Luna, mentions that he’s standing under the mistletoe. Harry warns her that ‘it’s probably full of Nargles’ but admits to her he has no idea what Nargles are. This time he doesn’t retreat, throws caution to the wind and kisses Cho, despite the risk of Nargles. [the film version is on Youtube here]
But what is a Nargle? No-one seems to know – they’re never explained and the implication, from other remarks, is that they are either non-existent (i.e. made up by Luna) or extinct or, possibly, just very elusive. In some of the (many) analyses of J K Rowling’s storylines online there are suggestions that maybe Luna made them up to avoid awkward situations – such as meeting Harry Potter under the mistletoe.
So that’s all clear. Nargles are possibly a fictional invention of a fictional character in a fictional world. But if they did exist, in the fictional world, they would infest mistletoe and steal things.
Nothing to worry about really. You’re far more likely to have one of the UK’s six mistletoe insects in your mistletoe. Of which my favourite is the Mistletoe Weevil Ixapion variegatum. Perhaps these are Nargles – they always look fairly mischievous to me. Though, being only a few millimetres long, it’s difficult to imagine them stealing much: