Spinach leafminers appear in spring, like young robins or daffodils, but nowhere near as welcome. The fly emerges, mates, and lays eggs on the underside of a leaf. Larvae hatch from the eggs, burrow between the leaf layers, and begin to eat, etching a serpentine path of destruction. Eventually the larvae drop to the soil, pupate, and produce the next of up to four generations, each of which lasts about 3 weeks. The leafminers' telltale trails can appear in the leaves of everything from tomatoes to ornamentals, but pose a real threat only to cool-season greens such as spinach and chard.
The first step in controlling these pests is monitoring. "Look for a small fly, more delicate than a housefly but larger than a gnat," says entomologist Ruth Hazzard. "Check for tiny rows of little white eggs on the undersides of leaves." Rub them off by hand. If you notice trails, immediately remove and trash—but don't compost—affected leaves. Additional control tactics:
- Exclude adult flies by using row covers.
- Encourage parasitic wasps by planting nectar-and pollen-rich flowers with small, shallow blooms, such as dill and yarrow.
- Cultivate the soil in fall to disturb pupae.
- Control weeds such as lamb's-quarter and dock that are known to be leafminer hosts.
- Rotate spinach, chard, and beet crops.
- Use a neem-based spray in severe cases. Neem acts as a repellant and also slows the leafminers' ability to feed, interrupting the cycle.