Have you ever noticed the leaves on your spinach and Swiss chard turning yellow and wilting, combined with dark spots and paper-thin spots on your leaves? Years ago when we first started growing spinach and Swiss chard consistently this issue started showing up. The first couple of years it really wasn’t a big problem so I kind of just ignored it, that was a BIG mistake. It turns out the problem is one that is actually quite common in our area, Leaf Miners!! Dealing with leaf miners in your garden can be a big challenge!
Back in 2011 when I attended my Utah State University Master Gardener class we had an entire evening where the subject was Entomology (the study of insects). This was perfect timing for me and I got my answer right in class. Dealing with leaf miners is something many gardeners have to do.
A leaf miner is actually the larval form of an adult bug that looks a lot like a small housefly. If you look closely in the photo above you will see some small flies, these are the adult form of the insect.
It is called a leaf “miner” because it actually burrows between the layers of the leaf and feeds inside the leaf, leaving an empty pocket behind. Leaf miners are one of the top 10 pest insects of vegetables in our state. Our variety of leaf miners is actually kind of boring. They hollow out the entire leaf leaving two paper-thin layers. In some areas, leaf miners are a little more interesting, they dig long fun tunnels in the leaves. Google “photos of leaf miners” some time and you can see the interesting patterns some dig!
You identify a leaf miner infestation by the large blotches in the leaves of spinach, beets, Swiss chard, and other related plants. The leaves will eventually dry up and die. Dealing with leaf miners is most difficult in plants you harvest solely for the leaves. The problem is a little less major in plants like beets where you can eat the root as well.
This is the aftermath of one leaf miner in this spinach leaf, I’ve separated the two halves of the leaf in this picture so you can see what they do.
You can also find the actual larva inside the leaves. Just pull off a leaf and feel around for a small lump.
Here’s a shot of one I caught in the act!!
Early identification can fix most problems. Look at the underside of the leaves for small white eggs.
Simply crush them to keep them from hatching.
Usually, leaf miners only have a couple of generations per year and are more common in the spring. But the conditions in my cold frames are ideal for them and they can have a second or third generation in the fall which is my current problem.
Spraying for these insects is not recommended. That usually makes the problem worse by removing the beneficial insects that prey on the larva. Usually, the local predators can take care of your problem for you if you give them some help by practicing good cultural methods. The following is a quick list of good cultural methods that will help you when you are dealing with leaf miners in your garden without spraying:
Crop Rotation – moving plating locations from year to year will help. If the adult flies emerge next year and can’t find the plants they like, the population will be greatly reduced. I think this is my biggest problem. Even though I rotate where my spinach and chard are planted each year the plantings are still quite close together. The leaf miner is also more prevalent in areas where spinach is overwintered.
Sanitation – keeping the area clear of plant debris is important. This is especially important with leaf miners. As they overwinter in the soil.
Crop Destruction – removing the infected leaves is a good control for leaf miners, but be sure to remove them completely and destroy them. Don’t put them in the compost bin. I think this is another part of my problem. Last year I didn’t know what the problem was and I will admit to just pulling some of the problem leaves off as I was harvesting and letting them lay in the bed. This allows the larva to finish growing and move to the soil to over winter. This year all the infected leaves are going straight to the chicken coop for a tasty snack for the chicks. If you don’t have this option be sure to throw all the infected leaves away.
Crushing the Eggs – This can be a long tedious process if you have a big infestation. Get out early and look for those eggs and crush them or scrape them off with your fingernail.
Use a barrier – Another effective method for preventing infestations is covering the crop with the lightest weight of a fabric row cover material. These lightweight row covers will keep the pests out (in this case the adult “flies” that lay the eggs), without blocking the sun for your growing plants.
We have found this to be the most effective protection against leaf miners. On the years that we get row cover over the bed as soon as the seedlings emerge, we have almost zero problem with leaf miners. This also cuts down the population for future years! Notice in the photo above, no leaf miners! This also keeps out aphids, but it’s not perfect for other pests like grasshoppers and snails.
Tillage – I’m not a big tiller because I don’t like what tilling does to the soil structure but in some cases, like this one, tilling will expose the bugs to the elements over the cold winter and help reduce the population.
So here’s my plan of action for dealing with leaf miners: Each year I’m going to remove all the infected leaves and destroy them and hope the plants can recover and produce for me in early spring. Each year I’m going to rotate all my spinach and related plants far out of the current growing area. I’m also going to watch closer in the spring and fall for the tell tale white egg masses on the leaves and destroy them before they hatch.
If you would like more information on dealing with leaf miners you can read a great fact sheet published by Colorado State University by clicking on this link.