This is my first year growing chard, because I didn’t figure out until last year that chard isn’t nearly as gross to eat as kale is. (I know, being a kale hater is like heresy in the Seattle edible gardening world.) That means that this year is also my first major run-in with leafminers. Leafminers are insects that feed between the upper and lower surfaces of leaves as maggots, and leave ugly brown tunnels in leaves. After enough mines are created, the tunnels no longer appear as distinct paths, and just look like brown, crinkly blotches, that are even grosser to eat than kale.
There are a few insects that cause similar looking damage, but the most likely suspect in chard or spinach leaves is the spinach leafminer, Pegomya hyoscyami. As adults, spinach leafminers are 1/4″ gray flies that lay eggs on the underside of leaves. Within a week, itty-bitty maggots hatch, and wriggle inside the leaves to feed. After another 2 or so weeks, the maggots are full grown and drop out of the leaves and burrow into the soil to pupate. Within a month, an adult fly emerges, and the whole cycle starts again. Once the summer is over, some leafminers will overwinter as a puparium in the soil, in order to start the cycle again the next year.
It didn’t occur to me to worry about protecting my plants from leafminers until they showed up and took out most of my chard crop. If I had payed closer attention to my vegetable beds this spring, I probably could have saved more of my chard by removing the first infected leaves I saw, since the leafminers are probably already in the their second generation this year. I didn’t catch the problem until early this week, by which time all of my chard plants had significant damage, and my spinach plants had so many eggs on them, that I harvested it all as baby spinach for salads (and washed it thoroughly before eating, because eating fly eggs is also pretty gross).
Besides removing infected leaves, next year I’ll plan on planting my spinach and chard in a different vegetable bed as part of my crop rotation, because of those puparium overwintering in the soil. Another way to prevent leafminer damage is to use a hoop house or floating row covers, which operate by covering the plants with something that lets light in, but keeps insects out. Floating row covers are made of white garden cloth, and hoop houses sometimes use garden cloth, and sometimes use white semi-transparent plastic. Both methods also help protect early crops from frost. I’m reluctant to go to the work of setting them up, though, because they also don’t let bees or butterflies in for pollination, and I tend to mix crops like flowering herbs, greens, and raspberries in the same bed. Hoop houses are also pretty ugly, and I consider my vegetable beds to be part of my overall ornamental garden. Leafminers can also be controlled by spraying neem oil, which is among the less alarming things you could spray on plants.
After removing all the infected leaves, and having mostly chard stubs left, I though about giving up on growing chard and spinach, but then my Territorial Seed Company winter catalog came, filled with seductive late-harvesting varieties, and I’d bet that chard will be making a reappearance in my fall garden.
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