Spinach and beet leafminers are two closely related species of early-season pests that cause damage to early greens. These insects attack crops and weeds in the plant family Chenopodiaceae, which includes chard, beets, and spinach, as well as several weed species including lambsquarters. Spinach leafminer (Pegomya hyoscyami) and beet leafminer (Pegomya betae) are very similar species in behavior, appearance, plant hosts, and damage and generally cannot be distinguished in the field. However, beet leafminers prefer laying eggs on beet leaves and spinach leafminer may also cause damage in solanaceous crops such as peppers.
Life Cycle
These insects overwinter as pupae in the soil and emerge around mid-May, mating and laying neat rows of eggs on the undersides of host leaves. After 3-6 days, larvae hatch from these eggs and burrow into the leaf. They become fully grown in just a few weeks and drop into the soil to pupate, though they may also pupate inside the leaf.
The entire life cycle is 30-40 days and there are 3-4 generations per season. These generations overlap and can cause continuous season-long damage to succession-planted spinach, beets, and chard. Typically, mid- to late-May, late-June and mid-August are peak activity periods. After August, pupae enter the overwintering phase and won’t emerge until next spring.
Identification
- Adult spinach and beet leafminers are small, gray flies 5-7 mm long. They are similar in appearance to many other flies and are difficult to identify definitively in the field.
- Eggs are small (<1 mm), white, oblong, and laid on their sides in neat rows on the undersides of leaves.
- Larvae are pale, white maggots which lack legs or a distinct head capsule. They can be seen by peeling the outer layer of the leaf to reveal one or several maggots.
Crop Injury
The leafminer larvae feed by burrowing between the layers of a leaf, eating everything but the outer layers, known as the epidermis. Early damage is a slender, winding ‘mine’, but these later expand and become blotches on the leaves. These maggots may migrate from leaf to leaf down a row.
In most seasons, the damage is minimal and the plants will outgrow it, leaving only early leaves with cosmetic damage. In other years, or other fields in the same year, the damage may be severe, and if the plants are hit early and growth is slow because of weather conditions, the loss may be great. This may be especially true when eggs on transplants in the greenhouse go unnoticed until planting in the field, resulting in infestations in row-covered crops.
Monitoring & Thresholds
Begin scouting susceptible crops in mid-May. Scout undersides of leaves for eggs and treat when they are first observed in order to target larvae as they hatch. Scout again 7-10 days after treatment, searching for eggs and the first tiny tunnels, to determine if follow-up treatment is needed.
Management
Cultural Control
- Control weeds which can serve as hosts of spinach and beet leafminers. These include chickweed, lambquarters, and nightshades.
- Rotate beet, chard, and spinach to new fields in the spring and during the growing season.
- Destroy heavily infested crops and crop debris before leafminers have emerged to decrease the risk and severity of future infestations.
- Use row covers or insect netting to exclude flies. Covers must be placed over crops before flies are active. Do not cover crops in fields where susceptible crops were grown previously, as adult flies may emerge from the soil underneath the cover.
- Avoid placing spring plantings near tunnels where winter greens were grown.
Chemical Control
There are effective treatments available for both conventional and organic growers. Use an adjuvant to improve efficacy unless the product label advises not to. Ensure good coverage of the lower leaf surface.
Labeled conventional materials include abamectin (e.g. Agri-Mek) and Proclaim (Group 6), Coragen, Exirel, and Verimark (28), Trigard (17), Venom and Platinum (4A), Pounce (3A), and Radiant (5). Of those, only Radiant is labeled for use on beets. Brigade (3A) and Harvanta (28) are also labeled for use on spinach. The diamides (Coragen, Exirel, Verimark, Harvanta) and neonicotinoids (Venom, Platinum) are systemic and some may be applied to transplants or to the soil. Radiant has some translaminar activity, particularly when combined with a penetrating adjuvant, and may be effective against larvae in leaf mines.
The most effective OMRI-listed material is spinosad (e.g. Entrust) applied before egg hatch. Like Radiant, Entrust has some translaminar activity and may be effective against larvae within the leaf. Improved control is achieved by adding a spreader sticker or insecticidal oil e.g. Suffoil or JMS Stylet Oil.
For current recommendations and information on production methods (including varieties, spacing, seeding, and fertility), weed, disease, and insect management, please visit the New England Vegetable Management Guide website.