From a Correspondent:
I have a rose bush whose leaves are being eaten. A neighbor told me that the culprit is called a leafcutter bee. How do I kill them so they don't kill my rose?
R., in Phoenix, Ariz.
There is nothing practical you can do to kill the leafcutter bees, but you don't need to worry about them killing your rose bush: they won't kill it. That's good news for both you and the bees, because leafcutter bees are an important natural pollinator, and we really don't want to kill them.
Pesticides generally work either by contact — they get on the target insect — or through ingestion: the target insect eats the pesticide directly or incidentally. Ant and cockroach baits are examples of direct ingestion, and systemic and topical poisons that are eaten when an insect sucks the sap or eats a part of a plant are examples of incidental ingestion.
The reason we can't kill the bees through ingested poison is because they're not actually eating the leaves; they're cutting out segments of the leaves to use for building their nests. We can't kill them through a contact pesticide because they're bees and they fly faster than we can spray them. While you could in theory stand in your yard with pesticide and spray everything that moves, I doubt you would have much luck, and your yard would be toxic as a result. Keep in mind that almost anything that can kill an insect is bad for you, too.
While leafcutter bees do cut holes in the edges of leaves (see picture), they rarely, if ever, do enough damage to compromise the health of the plant. Unlike grasshoppers and caterpillars, bees tend to take a small bit from a number of different leaves rather than destroy entire leaves en masse. The damage can be unsightly, but weighed against the bees' role in fruit and vegetable pollination, it's pretty minor.
And since there's really nothing we can do about them anyway, it's just as well to accept them as one of the quirky, interesting parts of living in the Desert Garden.
I hope this helps,