The webs often include what gardeners sometimes refer to as “beneficial insects“, which are species of insects that perform valued services like pollination and pest control.
Take for example the interesting fly in the photo. Xerox tigrinus, aka Tiger Bee Fly, is usually active in Maryland in July and August. Adult flys, like this one, are pollinators and will visit summer-blooming native plants.
The larvae, however, are parasitoids or predators of Carpenter Bees when they are the pupal stage. Adult female Tiger Bee Flies lay their eggs near the entrances to Carpenter Bee nests (the bee in this photo is near a nest entrance on a porch railing in Roland Park). When the eggs hatch, they climb into the Carpenter Bee nest.
This process may sound gruesome, but it is one mechanism for keeping Carpenter Bee populations under control.