Seed Maggots

Spinach and beet leafminers have been around in low numbers for the past few weeks, which is a couple of weeks ahead of schedule but have increased rapidly in the last 5-6 days. These leaf miners are found in beets, swiss chard and spinach but I usually find them more in swiss chard than spinach. Both of these leafminers are a type of blotch leafminer, creating irregularly shaped mines. These flies attack crops and weeds in the plant family Chenopodiaceae, which includes chard, beets, and spinach and the weed lamb’s quarters. Both fly species are very similar, but the spinach leafminer may also feed on Solanaceous crops such as peppers.

The unusually warm winter and early spring we have had up to now has allowed large populations of seed and root maggots to invade our vegetable fields. Some farms have been hit particularly hard in their onion, bean, pea or radish crops this season by maggots. These maggots include seedcorn maggot Delia platura (SCM), onion maggot Delia antiqua (OM) and cabbage maggot Delia radicum (CM). All three species overwinter in the soil as a maggot inside a brown pupal case (fig. 1). In March and April small, grayish-brown flies (fig. 2) emerge, which are usually SCM or CM, OM flies usually peak 2-3 weeks later. Adult flies are most active from 10 a.m. – 2 p.m. and are inactive at night, in strong winds or when temperatures are below 50o F or above 80o F. Adults live 2-4 weeks and females lay hundreds of eggs.

IPM Threshold Guide for Vegetable Crops