The Psilidae are a small family of flies in the order Diptera, commonly known as rust flies. This group contains around 4 genera and 38 species worldwide, with several found in the UK and Europe.

Rust flies are small, slender flies, usually yellowish or orange-brown in colour, which gives them their “rusty” appearance. They have clear wings, long legs, and conspicuous eyes, often seen resting on grass stems or low vegetation.

The larvae of many species develop in the roots or stems of plants, and a few are known as agricultural pests. The best-known example is the Carrot Rust Fly (Psila rosae), whose larvae tunnel into the roots of carrots, parsnips, and other related plants, causing discoloured, damaged crops.

Adult rust flies are most active in spring and summer, flying close to the ground in sunny, sheltered spots. They feed mainly on nectar and honeydew, and though small, they can often be seen flitting among grass and hedgerows.

In the UK, rust flies are most often encountered in gardens, farmland, and meadows, where they play both beneficial and pest roles—some helping decompose organic matter, while others can be troublesome to growers of root vegetables.