Regarding prevention, removing ALL infected fruitlets asap is the only effective control in my opinion. Books will suggest cultivation of soil under trees as a way to control the problem. All my pear trees are grown in soil that is cultivated and whilst this might have been one reason the problem has been slow to develop here, it hasn't prevented it from increasing in incidence. I've dismissed spraying as an option as it is unpractical to do all the trees, at least for the time being.
It should be possible to find and identify a large number of affected fruits on cordon-grown pears. The problem is that because there is so much variability in the way the grubs affect the appearance of the fruit they infect, it can be easy to miss on a variety that hasn't suffered before. It's also a bit demoralising to cut open slightly bumpy fruits only to find that they are unaffected and lose yet more fruit. So far there has been a fairly predictable pattern to the infection: Comic, Conference and Devoe had single fruits affected sporadically over the cordons (usually the earliest central fruits in the cluster); Morettini has whole spurs affected, and this was also the case with Fondante d'Automne a couple of seasons back. This year I have only found one fruitlet affected on the latter, no doubt I have missed some.
In contrast, my in-laws 15 year old Conference has had the entire crop affected for several years now, every single fruit turning black and dropping by the end of May, even the few not affected by midge larvae.