The larvae of many hover flies are beneficial in the garden, because they feed on aphids. Eristalis tenax, the drone fly, is one of our commonest hover flies, but its larvae aren’t so useful: they are the so-called “rat tailed maggots”, living in water and feeding on microscopic things they find there.
You might see them sometimes, in a pool, or a forgotten bucket of water, with their “tails” poking up to the surface of the water.
Nevertheless, even if the larvae aren’t particularly useful, the fly itself is still an important pollinator and a very welcome visitor.
Dandelions are great for attracting hover flies of many types, so it is worth keeping a few. If you’re worried about seeds, pick the flowers before they turn to seed-clocks and throw them in the bin, or on a fire, don’t put them in the compost heap!