If you grow leafy greens in your garden, it’s likely that you grow spinach. It can be a little (or more) disappointing to pop out to the garden to harvest some only to find that some of the leaves have damage in the form of whitish winding or blotchy patterns as shown below.

 

These are the handiwork of the spinach (Pegomya hyoscyami) or beet (P. betae) leafminers. On spinach, it’s most likely the spinach leafminer but their biology and damage is pretty much the same. Beet leafminers prefer to lay their eggs on beets if they’re available. In my garden, both plants are present so I think I’ve got the spinach species because the beets are not affected.

This pest overwinters in the soil as a pupa right near the plants it developed on the previous season. In the spring from roughly late April through mid May, adult flies emerge, mate and lay eggs. This is the best time to apply an insecticide if you’re going to because you need to intercept the tiny maggots between the time they hatch and the time they burrow into the interior of the leaves. This is a very short window of time. Scout for the clusters of tiny white eggs (visible to the naked eye) on leaf undersides.

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Spinach leafminer eggs. Photo: J. Boucher, UConn

 

Alternatives to insecticides for the home garden include crushing the eggs, removing leaves with eggs or mines/larvae and disposing of them in sealed bags in the trash, and crop rotation combined with the use of floating row covers for the next crop. Another option for infested leaf disposal is to bury them deeply in your compost pile where they will not survive.

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Spinach leafminer maggot (larva). Photo by J. Boucher, UConn

It’s important to keep in mind that the life cycle of the leafminers is only 30-40 days long so there can be 3-4 generations per year and they can overlap. If you have damage in your spring crop and are planning on a midseason/fall crop, move the spinach, beets and chard to a different area and cover prior to adult emergence with floating row cover to exclude the adult flies. Adults are tiny, hairy gray flies about 5-7mm long.

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Adult spinach leafminer (Whitney Cranshaw photo, Bugwood.org).

In beets, moderate damage will not affect the development of the root crop. This pest is most important in plants grown for edible greens. Having said that, severe damage in beet leaves can result in poor root development.

By Joan Allen