Bradford pears (Pyrus calleryana of Callery pear) has to be one of the champions of over-planted ornamentals especially in the tree category. They do grow quickly, and then because of their anatomical weakness, they fall apart just as quickly. They do have nice foliage, dense, dark-green, so don't expect grass to grow underneath one. And they do have a nice fall foliage color, and quite a nice lacey white flowering display. Locally they just began to flower, so it came as no surprise when the phone rang and someone was asking TPP the usual question: What smells so bad? In flower Bradford pears stink; they smell just awful. It's hard to describe some odors especially in the broad category referred to as "rank". This term is used to describe a set of offensive (to the human sense of smell) odors. The pears odor is strong and reminds TPP of the odors emitted from sewers in Bangkok. No question about it; the smell is disagreeable. This seems to puzzle many people, but these people have never worked with flowers or their biology. Flowers do not exist to please our senses, and even though these flowers are pretty enough, they do not use the same pollinators as say apples which have a very similar flower, both being in the same sub-family of the rose family. As is often the case, it's still pretty cool weather because Bradford's flower in the early spring, and one of the most reliable pollinators of the early spring are syrphid flies. 
Here's an image if you aren't familiar with them; you probably thought they were bees, but they hold their wings differently. Now when it comes to odors, bees and flies have very different likes and dislikes. There's a reason for that old saying, "Whew, that could gag a maggot!" Flies look for rotten things to use as a brood substrate for their offspring, although in the case of the pear, they don't get that, although they probably do get some nectar as a reward. And now that we've gotten this far, why is such a flawed tree so widely planted? It's got three strikes against it: dense shade, weak limbs, and it stinks. Best thing that ever happened around here was a very early, wet, heavy snowfall in late November when the Bradford's were still holding their leaves and the snapping of the trees sounded like fire works. A walk around just a few blocks and over 40 Bradford pears were just so much fire wood; such a good start!