These bees prefer sweaty people—over most animals—because the human diet usually is so salty that their perspiration is saturated with the essential nutrient, experts said. Yet most people never notice when the tiny bees alight on a bare arm or leg.

As it turns out, Mr. Ascher and his colleagues are discovering New York City is a hive of activity.

By latest count, about 250 species of native bees are known to nest in sidewalk cracks, traffic median strips, parks, and high-rise balcony flower pots—more perhaps than any other major city in the world, several entomologists said. In Prospect Park alone, at least 90 species of native bees flit from flower to flower among the park’s sun-dappled golden rod, dandelions and dogwood.

“For certain species, the city is as good as or better than a natural area,” Mr. Ascher said.