Just when you thought it was safe to return to the garden, they're back.

A wave of dog day cicadas, cousins of the 17-year cicadas that created a sensation this spring when they rose by the millions from their long hibernation, are emerging from their underground cocoons. They are the relatives who visit every year.

"They usually come out around the Fourth of July. They just missed the other ones by a week," said Richard Froeschner, an entomologist with the Smithsonian Institution.

The dog day cicadas, so named by the early American settlers because they appear in the steamy months of July and August, each live three to four weeks and will be around through September. They create a high-pitched grating song that some may find annoying, but they cause no damage to trees or other vegetation, entomologists say.

"If they did harm, they would have harmed trees out of existence," said Froeschner. "They are not going to destroy their own food source."

Although there is a strong family resemblance between the dog day cicada, also known as the common cicada, and the recently departed 17-year variety, there are some key differences. For one, dog day cicadas are quite a bit larger.

"If you put a dog day cicada and 17-year cicada side by side it would be like comparing a Cadillac to a Volkswagen," said Stanton Gill, an entomologist with the University of Maryland Cooperative Extension Service.

Another big difference is that the dog day cicadas are more high strung. And with good reason, say entomologists. They have to contend with the cicada killer wasp.

These wasps sneak up on cicadas and give them a paralyzing sting. After hiding her prey in an underground nest, the wasp lays her eggs on the cicadas' bodies. When the larvae hatch, they feed on the stunned cicadas until they retire for the winter.

"Sometimes you can hear the cicadas singing in a tree and suddenly the buzzing turns into a shriek. That's a good indication that the wasp has found a cicada," Froeschner said.

Although they appear annually, the dog day cicadas actually take five to seven years to mature.

Sometimes, for reasons scientists are unable to explain, those cicadas appear a year earlier or later than normal and establish populations which then mature in five to seven years. The unexpected arrivals have resulted in staggered cycles and annual appearances.

There are at least four species of dog day cicadas in this area, each with its own song.

"To try to describe the songs is like trying to tell the difference over the phone between a violin and viola," Froeschner said.

The dog day cicadas tend to spend their short time above ground much the same way the 17-year variety did. The males sing to attract the females with whom they mate. After the females lay their eggs in tree branches, the adult cicadas die.

The newly hatched cicadas drop to the ground, then make their way underground where they feed off tree roots and wait five to seven years before emerging.

Unlike their cousins, the dog day cicadas appear in fewer numbers, a fact that dismays Douglass Miller, an entomologist with the Department of Agriculture Research Center in Beltsville who found the 17-year cicadas a tasty treat.

"I'd love to try eating {the dog day cicadas}. But getting the numbers to cook up a batch is difficult. They stay up in the trees," Miller said. "They are larger than the periodic cicadas and ought to be more wonderful."

Miller isn't the only one who appreciates the latest wave of cicadas.

"We enjoy them," said Capitol Hill gardener Frank Bonora. "They remind me of lazy hot summers in the District."

Henry Allen, an avid gardener and past president of the Men's Garden Club of Montgomery County, said: "I enjoy watching my mocking birds chase them. It's great fun. Besides they do little or no damage. Why fight them?"