Controlling Leaf Miners

The spinach leaf miner has been a scourge in my garden this summer. While I’ve been snipping and destroying affected leaves, which is the best tactic once your plants are under attack, I’ve also been conducting an experiment to see if I can avoid the pest.

Leaf miners attack the leaves of spinach, beet and Swiss chard, and their presence can be devastating and unsightly. At our Master Gardener booth at the Ballard Farmers Market this summer, we have had more questions about leaf miner damage than any other problem. Clearly, this is a pest that has flourished in northwest Seattle this summer.

Here’s what the leaf miner (Pegomya hyoscyami) can do to a leaf:

Leaf miner damage

That photo is a bit dramatic, because of a recent rain that rotted the damaged parts of the leaf.

Here’s perhaps a more typical view of the damage:

Leaf miner early damage

What you see is the result of the pest larvae burrowing into

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the leaf and mining their way through the cells. They arrive at the plant via a small fly, which lays its eggs on the underside of the leaf:

Leaf miner eggs

One way to thwart this pest is to regularly search for those eggs and destroy them when you see them. But if you love beets and chard like I do and have a lot of plants, this method is difficult. So I tried another way.

This summer I started a batch of Rainbow chard in a large pot and covered the pot completely with floating row cover, the spun-bonded polyester fabric that is so useful to us cool-season gardeners.

As the plants grew, I had to prop up the material to get a sprayer for my watering system into the pot. I used interlocking wire flower stakes to hold it up. Here’s what it looks like:

chard in pot with floating row cover

And here it is with the floating row cover removed:

chard in pot with row cover removed

Although the chard is planted rather thickly and hasn’t achieved its full size yet (I’m thinning as I eat it), I found that I could completely avoid the leaf miner with the floating row cover. I have been monitoring the pot regularly and have seen no damage. Just six feet away in the adjoining bed, I had such heavy damage to a row of baby beets that I had to harvest them prematurely.

As I continue my fall planting of beets and chard, I will again cover the plants with floating row cover. I’ll also plant the new crop far away from the area where the plants got hit the hardest. Crop rotation and floating row cover are tried-and-true non-toxic ways to reduce pest damage in the vegetable garden.


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