Larvae mine their way through leaves creating blisters that often look like meandering tunnels. In spinach and chard affected leaves are not marketable. Affected beets may not be marketable with tops, but damage is rarely high enough to defoliate to the point of effecting sizing up of beets.
Leaf miner in spinach.
Leaf miner eggs.
Leaf miner damage is caused by the legless yellow to white larvae which burrow between the layers of the leaves as it feeds. The mature larva cuts a hole in the leaf and drops to the ground to pupate. It emerges 2-4 weeks later as a fly. This fly lays small white eggs, generally on the underside of the leaf. Eggs hatch in 3-6 days. Often there are several larvae within each mine. In Pennsylvania there are several generations per year: taking about 30-40 days per generation Leaf miners over-winter as pupae in the soil or plant debris and emerge as adult flies in the spring.
Either the spinach leafminer or the beet leafminer cause this damage, and both have similar lifecycles. Both were introduced from Europe, probably in the 1800's. Many weeds serve as hosts, including lambsquarter, pigweed, henbane, and nightshade.
Early detection is important. Check young seedlings weekly for mining on the cotyledons and first true leaves. Look for clusters of small white eggs, mines and hatching larvae. Examine ten plants in ten locations. Be sure to examine the undersides of leaves.
- Weed control helps reduce populations. Historically, these have been more of a problem in gardens and small plantings, as opposed to larger weed-free fields. Your leaf miner problem may be a weed problem! If you are harboring high levels of lambsquarter, amaranth and other weed hosts you will likely battle leaf miner during peak flight regardless of chemical controls.
- If detected early in smaller plantings, removing infected leaves can suppress the problem.
- Rotation is important away from spinach, chard, and beet hosts. Pupae overwinter in soil and crop debris of host fields, including weed hosts.
- Many naturally occurring parasitoids and predators attack leaf miners. Consider whether your pesticide use might be reducing parasitoid populations and encouraging outbreaks.
- In smaller plantings use floating row cover to prevent egg laying.
- Till the soil in early spring to disrupt life cycle and kill over-wintering pupae.
- Most pesticides are ineffective against larvae because the miners are protected inside of the leaf. Spinosad/Spinetoram products (Entrust, Radiant) have activity if residues are present on the leaf surface.
- Systemic pesticides, either through the soil or translaminar activity to penetrate the leaf can be effective. Care must be taken (accurate rates, and follow days-to-harvest interval) to avoid residues in the marketed product. Products in this category include chlorantraniliprole (Coragen), abamectin (Agri-Mek), cyromazine (Triguard), dinotefuran (Scorpion, Venom).
- Efficacy improves with the use of an adjuvant.
Production Guide for Organic Spinach
Commercial Vegetable Production Recommendations