It seems like they have arrived earlier than normal, but Cicada Killers are beginning to emerge.
These large wasps emerge in the summer, males first, then females. The males aggressively protect a small patch of ground. I have seen them fly up/out at people, birds, even cars! They are waiting for the females to emerge and want to be the first to mate when they emerge. Even though they put up an aggressive front, they are incapable of stinging. Their 2 inch length and wasp appearance does cause distress in many people though.
The females emerge a week or more after the males and are even larger. They have the ability to sting, but rarely do. They are hunters of cicadas, with the Dog-day cicada being their main target. They will sting a cicada, carry it back (the cicada may be many times larger/heavier than the wasp) and bury it. An egg that will become a male will get one cicada, where a female egg will get 2-3 (since they are larger). When the egg hatches, the larvae has a fresh meal as the cicada is paralyzed by the sting, but not killed. The burying of these cicada creates large, unsightly mounds in lawns.
Treatment strategies are difficult with this insect. The males can be knocked down with aerosols or other residuals, but it is hard to get to all of them and it can look funny to try and chase one down across a lawn. The females are similarly tough to treat, but their burrows can be treated. A dust formulation is the best for these burrows, with the treatment plan designed to kill some of the females as they return to burrows with additional cicadas or to kill the eggs/larvae in the burrow so there are less emerging the following season.
These are a tough insect to deal with when they are in full season. Even though they look like the deadliest wasp around, stings are very rare--there are even some photos on the web that show female Cicada killers landing on peoples' hands. The unsightly mounds can do a number to a lawn as well.