How do I fight fruit flies on my pear tree?

Ugh, the fruit fly is such a pain, honestly. So, apart from the yellow traps (which do help anyway) I'd recommend picking up the fallen fruit right away from the ground because that's where they breed, then you can try spraying with Bacillus thuringiensis or mineral oils every 7-10 days from June onwards - last year I met this guy who delivers organic fruit and he told me it works well if you're consistent about it. Another thing someone suggested to me is covering the fruits with little paper bags when they're still small, like in June, so the flies can't even touch them, I know it's kind of a hassle but it really works.

The yellow traps are mainly useful for monitoring the infestation, they're not enough on their own to get rid of it. What actually works is using fine-mesh netting to cover the fruit starting from June, before the fly lays its eggs, or regularly collecting and destroying fallen and damaged fruit (the larva develops inside them). If you want to avoid chemicals, you can also try protein baits to attract and kill the adults, like yeast-based ones, or do spinosad treatments which are pretty natural. Unfortunately without some more decisive measures it's hard to save your whole harvest.