Onion fly

Delia antiqua

1. Identification

• Common name: Onion fly
• Scientific name: Delia antiqua Meigen
• Order: Diptera
• Family: Anthomyiidae

2. Pest description

• Adults similar to small houseflies, 5–7 mm, greyish body
• White, cylindrical, apodous larvae, feed on the internal tissues of the bulbs
• Brown pupae, formed in the soil
• White, elongated eggs, laid at the base of the plants or in the adjacent soil

3. Main hosts

• Onion
• Shallot, leek and garlic
• Other Allium species can be attacked

4. Symptoms and damage

• Sudden wilting of young plants
• Yellowing and curling of the leaves
• Internal rotting of the bulbs due to larval feeding
• Perforations and galleries inside the bulb
• Greater susceptibility to secondary infections by fungi and bacteria
• Significant losses in nurseries and young crops

5. Biological cycle

• Two to three generations per year, depending on temperature
• Adults emerge in spring
• Oviposition at the base of the plants or in the soil
• Larvae feed for 2–3 weeks
• Pupation in the soil, where they can overwinter
• Emergence synchronised with periods of higher humidity and moderate temperatures

6. Monitoring

• Yellow chromotropic traps for detection of adults
• Observation of wilting symptoms and damage to the bulbs
• Sampling of young plants to detect larvae
• Intensive monitoring in spring and early summer
• Recording of favourable conditions (high humidity and mild temperatures)

7. Management measures

• Cultural: crop rotation with non-host species; destruction of crop residues; sowing in periods less favourable to the pest; use of insect-proof nets; adequate soil drainage.
• Biological: conservation of natural enemies; application of entomopathogens in the soil.
• Integrated protection: continuous monitoring; targeted treatments when thresholds are reached; use of attractants and traps; adequate irrigation management; judicious application of authorised insecticides.

Bibliographic references

• EPPO Global Database – Delia antiqua
• CABI Invasive Species Compendium – Delia antiqua
• Finch, S. (1989). Ecology and control of onion fly
• Vernon, R. S. (1999). Monitoring and management of Delia spp. in Allium crops