Pests and diseases : Onion fly

Plants affected

• Mainly Onions but other members of the Allium family can also be attacked.

About Onion fly

• Adult Onion Flies emerge from pupae that have over-wintered within the soil.
• Pupal cases are brown, oval shaped and about 10mm long.
• Onion flies have several generations a year, the first generation usually emerges about May/June time.
• The first generation of adults to emerge are able to reproduce within a few days and females then lay batches of eggs onto or nearby onion seedlings.
• The eggs laid are small, white and elongated, with darker stripes that run longitudinally.
• Second and third generations attack the onion sets which will have developed from the seedlings.
• The damage is caused not by adult flies but by their grey, legless maggot-like larvae (8mm long) feeding on the onion sets and moving from one to another in the soil.
• Adult onion flies are similar in appearance to house flies however they are grey, slightly smaller and have longer legs.
• Larvae can often cause secondary damage such as rotting when they bore in and out of the roots.

Treatment

Chemical

Products containing the following chemical ingredients are all effective on Cutworms

• There are no approved insecticides currently available to amateur gardeners.

Note: It is important to read manufacturer’s instructions for use and the associated safety data information before applying chemical treatments.

Organic

• Regular observation of crops to check for damage or infestation and remove any affected plants immediately and incinerate.
• Keep weeds under control that could be alternative host plants e.g. dandelions.
• Crop containment e.g. Fleece barrier to stop flying adults, or grow in poly- tunnels.
• Avoid planting in soils that have un-rotted organic matter present.
• Plant sets instead of seeds and avoid planting in rows if possible to stop maggots moving from plant to plant.
• Avoid sowing at peak Onion Fly breeding times.

Prevention

• Very efficient checks of the crop for any signs of infestation, any prompt removal of any plants affected, use sticky traps to monitor numbers of adults using white and yellow traps.
• Use biological pest control species to control numbers, such as Predatory Flies – Coenosia tigrina, Wasp sp. – Aphaerata pallipes, Beetle sp. – Aleochara bilineata or the Fungus – Entomopthora muscae.

Original Article