Why does your Onion Bonus F1 have tiny black flies? Diagnose the cause and fix it with our step-by-step guide.
The larvae of these flies live in the moist organic matter of the soil and feed on fine root hairs, which can weaken the developing onion bulb. The adults are attracted to the high moisture levels often required for onion germination and early growth.
While typically associated with decaying fruit, certain small flies are attracted to the fermenting sugars if the onion bulbs have begun to rot due to fungal pathogens like Botrytis. They are scavengers that frequent high-moisture vegetable beds.
The adult flies lay eggs at the base of the onion plant; while the 'tiny black fly' appearance might be a misidentification of the slightly larger Delia adults, their presence is a direct precursor to maggot infestation. These larvae tunnel into the bulb, causing significant damage.
Your plant might also be experiencing one of these issues:
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