Onion fly (Delia antiqua)
The larvae of the onion fly, also called onion maggot is a major pest of onions. The maggot is small (about 8 mm in size when fully grown), white-cream coloured. It eats the lateral roots, then tunnels into the taproot and sometimes bores into the base of the stem. Attacked leaves wilt and the leaves turn bluish. The plants become shrivelled or eventually die. The maggots feed just above the base of seedlings killing them. A maggot can attack several seedlings in succession. This causes poor plant establishment resulting in many gaps in the field.
The maggots are also found inside developing onion bulbs. Their feeding exposes the plant to infection by diseases such as bacterial soft rot. Pupae are light to dark-brown in colour, and about 7 mm in length. Pupae are found in the soil near the base of the plant. The adult is a brownish grey fly, somewhat smaller than house flies. When at rest, they keep their wings folded one over the other. Adult flies do not cause damage. They lay eggs in the soil surface near the germinating plants.
Onion maggots are adapted to cool, wet weather, so usually they are less of a problem during hot dry periods. They prefer soils heavy in organic matter. The onion maggot attacks plants related to onion such as leeks, shallots and garlic.
- Avoid planting in soils that are high in undecomposed organic matter, such as fields just coming out of pasture or in very weedy conditions. Flies prefer to lay eggs in soil that is moist and with high organic matter. Do not plant onions unless the plant residues are dry and completely decomposed.
- In soils amended with animal manures, allow adequate time for the manure to break down before planting.
- Avoid planting successive onion crops. Practise rotation with crops not related to onions.
- Keep onion fields well separated. Onions grown in the season following an attack by onions flies should be sown as far away from infested land as possible.
- Remove and destroy infested plants and burn them.
- To prevent an infestation with onion flies, carefully plough-under crop residues immediately after harvest.
- Turn soil to destroy pupae.
- Powdered hot pepper or powdered ginger placed around the stems helps when the onion fly population is moderate.
- Neem-based products have a deterrent effect on egg-laying.