PesthighApr–Sep

Onion fly

Delia antiqua

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How to spot it

The onion fly (Delia antiqua) lays its eggs on the leaf sheaths of young onion plants, and the larvae then tunnel into the bulb, the neck and the roots. Affected plants show wilting, yellowish leaf tips, barely grow any further and can be pulled out of the soil easily. Inside the bulb itself you will find whitish maggots along with a rotten, often foul smelling consistency. Young plants and freshly planted onion sets are especially at risk from May to July, when the first generation of flies is on the wing.

Prevention

Stick to a strict crop rotation: do not grow alliums in the same spot for at least 3 to 4 years, since the pupae overwinter in the soil. Mixed plantings with carrots or strawberries confuse the fly on its approach through their scent. Cover the beds with fine mesh crop protection netting or fleece right after planting, so that the flies cannot lay their eggs in the first place. Remove wilted and infested plants from the bed immediately, bulb and all, and dispose of them with the household waste rather than on the compost; that way you break the development cycle.