What to do with leafminers on chard

I struggle with leafminers every time I plant chard.  This fall is particularly bad, and if I didn’t diligently tackle them this week, I am pretty sure there would have been no chard left for me to munch on.


The Pest - Leafminers

The innocuous looking leafminer fly.

Leafminers are the larvae of an innocuous looking fly.  The fly buzzes around the yard, finds your delicious harvest and lays eggs on the underside of highly susceptible chard, beets, spinach, and or/other desirable plants.  The white, oblong eggs are laid in neatly grouped rows.

White leafminer eggs laid in groups on the underside of leaves.

The Damage

Leafminer damage from a mining larvae.

I don’t usually notice I have leafminers until I see the damage to the chard.   Leafminers tunnel between the two exterior layers of the leaf (the epidermis) and eat everything soft and delicious in between.  As they mine out the middle of the leaf, they move forward and leave a hollowed out trail.  The trails, or mines, become wider and more damaging as the larvae grow.  Large sections of the leaf will become hollowed out with a paper-thin epidermis layer that is white or black.  The plant can usually survive all this damage, but the individual leaves lose their culinary appeal.

Major leafminer damage from larvae.

The Solution

Leafminers can be controlled by removing the eggs on the leaves before the larvae hatch, and by squashing the larvae living between the epidermis layers. First, cut out the leaves that are too damaged to be worth saving. Next, inspect the underside of each remaining leaf and rub off any eggs with your fingers.  Leafminer eggs can be stubborn and it can be easy to damage the leaf in an effort to remove them.  After removing the eggs on an individual leaf, look for mines.  Pinch the mine along its length between your fingers and you will squash the larvae living in between.

Egg removal and squashing the larvae are simple, but tedious activities. This week I spent a minimum of 30 minutes trying to eradicate all the leafminers and eggs on my chard.  I thought it was time well spent until a few days later when I inspected the leaves and was shocked to see the number of new eggs that had been laid. 

An example of a barrier screen.

I believe the best way to control leafminers is to prevent the flies from laying their eggs on the plants in the first place.  This can be done with a Reemay cloth or screen that creates a barrier, but also allows light and air to penetrate.  Check back to see my upcoming blog entry with instructions on how to construct an easy to build, reusable barrier screen for a raised bed. Building the screen takes an initial investment of time, but it will make controlling the leafminers an effortless task in the future.

Keeping leafminers from damaging your plants is worth the effort.  I encourage you to peel open the epidermis layers of your damaged plants and inspect the leafminer larvae inside the mines.  They are fat, pale white, and not an appetizing looking grub. I don’t know what kind of flavor they can contribute to a meal, but if you are like me, you'd rather not find out.