Biological control of onion fly

Encourage natural predators like birds, hedgehogs, and ground beetles into the garden. Hedgehogs like dense undergrowth and grasses for hiding and nesting. You can also build or purchase hedgehog homes. 

A mix of pathogenic nematodes can be purchased from biological control suppliers. These microscopic organisms enter caterpillar bodies and infect them with a bacterial disease. Water nematodes into the soil in late spring.