Onions and related crops are attacked by the onion fly, D. antiqua (Mg.), and by the bean seed flies, D. cilicrura (Rond.), and D. trichodactyla (Rond.). The eggs, larvae and puparia of the three species are similar in form, and the only morphological difference so far observed is in the number of processes of the anterior spiracles of the larvae. In D. antiqua the anterior spiracles have 10–13 processes; in D. cilicrura and D. trichodactyla, the immature stages of which cannot yet be distinguished from each other, the anterior spiracles have 5–8 processes.

Onion flies and bean seed flies have different habits and consequently the circumstances of their attacks differ. The onion fly is a specific feeder and its attacks are likely to be serious in spring and early summer. The bean seed flies are general feeders and attack on onions in spring is unlikely because the preparation of the soil, which attracts the flies, is usually completed before they emerge from the over-wintering puparia. Bean seed fly attack on onions is likely to take place in August when onions for bunching are sown. The preparation of the seed bed attracts the flies of the third generation from the surrounding crops and the soil may become heavily infested with larvae which subsequently destroy the germinating seeds. In August and September it is unlikely that young crops will attract onion flies from the mature crops on which they are established.