Last year my spinach crop was mostly decimated by the time I figured out it wasn't blight but a creature that was eating it. Once you know what to look for, the signs of leaf miner are obvious and unmistakable.
|The trail of a leaf miner in chard.|
Leaf miner can refer to a bunch of different creatures that behave similarly: they lay their eggs on leaves, and when the eggs hatch the larvae (little worms) eat a path through the leaf, munching between the layers until they are ready to drop out into the ground to turn into moths or flies and start all over again. The ones eating my spinach (drum roll)...are most likely spinach leaf miner. Now I know better than to let them get out of control. Once a plant is established, and if you don't mind a few blemishes (and the memory of a gross larva) in your greens, then leaf miners aren't a big deal. But in my small home garden they made a very big dent in my spinach and chard crop last year, especially when the plants were young.
|Leaf miner damage to a beet seedling|
So now I keep them in check by rubbing the eggs off the leaves before they hatch. The eggs are easy to spot - tiny tight rows of tiny white eggs on the backs of the affected crop.
|A row of leaf miner eggs on the back of a chard leaf|
I have raised beds so I just take my coffee outside in the mornings and sit on the walls while turning over leaves and rubbing off the eggs. There are usually a couple patches of eggs on the backs of each leaf of my spinach, chard, and beets every other day or so, so that would be a lot of little grubs eating up my food if I didn't get after them. Maybe after a season or two of management they'll be less prolific, but I don't mind the task.