A metallic-green beetle has arrived, posing a threat to ash trees — and the people who live near them.

That is the conclusion drawn by scientists studying the devastating effects of the emerald ash borer (EAB) in the United States. The exotic invasive beetle, first detected in Michigan in 2002, has laid waste to more than 100 million ash trees in at least 15 states, including Maryland and Virginia. The insect’s larvae feed on the inner bark of all 22 species of native ash trees, killing almost every tree infested within two to five years. The United States has about 7.5 billion ash trees. In some forests, more than half the trees are ash.

The rapid disappearance of such an abundant tree has provided a unique opportunity for foresters, statisticians and epidemiologists to see how tree loss affects humans by comparing changes in human mortality rates before and after the demise of the ash in almost 1,300 counties.

A study in February’s American Journal of Preventive Medicine showed that deaths from cardiovascular and lower respiratory illnesses rose as ash trees vanished. The study found that the EAB’s effects can be linked to more than 21,000 deaths — an additional 24 deaths per 100,000 people every year, a 10 percent increase in mortality for those diseases.

Although the authors did not detail any direct cause-and-effect relationships, they did cite previous studies demonstrating the health benefits of trees: They improve air quality, moderate temperature and provide opportunities for physical activity; trees are psychologically soothing and act as buffers for stress; a walk through the woods reduces heart rates and lowers cortisol levels; children living on tree-lined streets are less likely to have asthma.

Ash is the most common tree in Baltimore, comprising more than 10 percent of the metro area’s canopy: 6.5 million ash trees. In the District, the urban canopy is only about 2 percent ash — but that still adds up to more than 51,000 trees, according to the nonprofit organization Casey Trees.

This month, half-inch-long adult EABs will emerge from D-shaped holes in infested ash trees. They will feed on ash leaves and then lay eggs in crevices of the tree’s bark. Larvae will hatch to burrow under the bark, carving out meandering tunnels that will girdle and inevitably kill the tree.

Although Manchurian ashes from the beetle’s native range in East Asia have evolved chemical defenses against the borer, most American ashes are defenseless. However, the blue ash, a species from the Midwest, appears to be attacked much less frequently than other native ash species and might be able to persist in what researchers call “aftermath forests,” where other ash species have disappeared so completely that even the beetle is absent.

People finding EABs are encouraged to report the sighting. In Maryland, call 410-841-5920; in D.C, call 301-313-9327; in Virginia, 804-786-3515. An entomologist might be dispatched to confirm the sighting, but it’s up to the property owner to decide the fate of the tree.

Relatively healthy trees can be protected from beetles with expensive systemic pesticides applied every two years. But extensive infestations can prevent trees from transporting the chemicals up through the trunk and out into the limbs. Cutting down dead or dying trees is the responsibility of property owners, but wood cannot be transported outside of quarantined areas, which locally includes Maryland counties west of the Chesapeake Bay, all Virginia counties and the District.

Biological-control measures have been deployed in several states to try to check the spread of the beetle. Biologists have introduced several species of tiny parasitic wasps from East Asia that lay eggs exclusively on EAB eggs and larvae. The wasp eggs hatch into predatory larvae that can doom a beetle’s prospects.

In Michigan, two of the three stingless wasps are now widely established, parasitizing about a third of all EABs, said USDA research entomologist Jian J. Duan. In Maryland, where wasps have been released for the past two years, parasitism rates are at about 10 percent. “More releases and additional time are still needed to allow those introduced parasitoids to establish an expanding population,” Duan said.