Purple boxes hanging in trees?

I've noticed a few purple colored boxes hanging in trees by roadsides. I've seen these in Fairfax County. What are they?

The purple boxes are for trapping insects, specifically the emerald ash borer, an invasive green beetle from Asia that is rapidly destroying ash trees in a number of states from Michigan to Virginia. In 2007, an infestation was discovered in Herndon.

Source: washingtonpost