You can often see hoverflies zipping in and out of flowers in your garden. They approach a flowering shrub or group of flowering perennials and hover around seemingly deciding which flower to feed from. A nice part about hoverflies is that they frequently land on flowers to rest providing great photo opportunities for even the clumsiest and least patient phone-wielding gardener or child. Hoverflies are often mistaken for bees. This is called Batesian Mimicry after Henry Walter Bates who studied butterflies (among other things) in the Amazon and first described the phenomenon of harmless species mimicking unrelated harmful species as a form of protection from predators. In this case, many hoverflies, which don’t sting, mimic bee species that do making predators think twice before grabbing them.
The superficial similarity continues since hoverflies also pollinate flowers, though not always as efficiently as bees. Hoverflies visit flowers to feed on nectar or nectar and pollen depending on the species. This gives them the energy and nutrients they need to reproduce.
Reproduction is where hoverflies and bees diverge. (Evolutionarily bees and flies diverged a long time ago during). Most hoverflies have free-living predatory larvae. Hoverfly adults lay eggs on plants near aphid colonies. The maggots move within the aphid colony grabbing aphids with their mouths and eating them. These are very easy to find if you want to see them in (slow) action. Look at milkweed, tulip poplar, any plant with a bunch of aphids. Look closely among the aphids and you will often see green or yellow hoverfly maggots.
Hoverflies can be valuable for biological control of aphids in crops like lettuce and grains on which aphids are common pests. Hoverflies can fly far into crop fields to home in on aphid colonies and lay eggs. A lot of research has investigated ways to attract and conserve hoverflies and other aphid predators like lady beetles in crop fields by planting flowers.
Even urban yards can have great hoverfly diversity. As you might expect the best way to attract hoverflies is by planting flowers. But remember flowers are not all they need; they also need aphids for the maggots to develop. Maintaining a “pest free” yard reduces the abundance and diversity of all the predators and parasitoids that rely on those herbivores as food. So when you see a few aphids don’t go nuts. Think of them as food for lady beetles (which also won’t lay eggs without aphids present), hoverflies, minute pirate bugs, bigeyed bugs, lacewings, parasitoid wasps, aphid midges, and hundreds of other insects. Expecting to conserve charismatic insects like hoverflies and lady beetles while eliminating aphids and other herbivores is like trying to conserve lions without gazelles, water buffalo, and zebras.