© Rasbak

Type: Pests


Onion fly (Delia antiqua)

The onion fly looks similar to a house fly but is grey and slightly smaller. It overwinters as a pupa in the soil and emerges around May/June, mating quickly and laying eggs on or around onion seedlings. The white maggots then burrow into the bottom of the bulb, where they grow to 8mm or so long, as they carve out tunnels. They will travel from one onion to another as they grow and after about three weeks, move back into the soil and pupate. Three weeks later, the next generation hatches leading to continued attacks, albeit of decreasing severity, from July to September. Growth is stunted and badly affected plants may die. The tunnels are unsightly but are often still edible with a bit of trimming. However, the burrows often lead to secondary infections and affected onions will not store well.

Symptoms

  • Yellow wilting foliage (outer leaves first)
  • Young plants may die
  • Growth of older plants is stunted and bulbs may rot
  • The maggots, or at least their tunnels, are easy to see of an affected bulb is lifted

Prevention & control

  • Autumn digging will expose pupae to predators
  • Sow very thinly and remove any thinnings as the smell will attract the fly
  • Grow under insect mesh
  • Pull and burn affected plants immediately to limit spread
  • Grow sets rather than seeds
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Onion Fly (onedot3 - 12:10 03/05/2013) Report inappropriate tip
Try growing onions with mint to confuse and deter onion fly
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