Although there are some newspaper articles with good information on these moth larvae: basic biology and how to deal with them, it is a shame that sprays have sometimes been used due to complaints of aesthetic nature, since they also kill non-target insects, including predators and parasites (that help bring the populations back down again in the longer term).
If the caterpillar tents are not too high, I use a pole pruner to cut down the tent webbing and dispose of it. This is best done early or late in the day, when the caterpillars are concentrated in the tent shelter. Spraying with a hose I have found to be ineffective.
If left unchecked on vulnerable shrubs or trees, foliage can be completely stripped. The “good” news is that all those droppings and dead caterpillars end up in the soil as fertilizer to help with the growth of a new flush of foliage. These pictures were taken on June 5, 2012, at Piper’s Lagoon in Nanaimo:
In my immediate neighbourhood, we are relatively unscathed: just a few tents, although quite a few individuals are found wandering about chewing on the roses etc. Those get a snip with a gardening instrument. The rest will no doubt be predated by wasps etc.