Leaf Miners
by Rob Sproule

Have you noticed squiggly brown lines appearing on your birch, lilac, and vegetable leaves?   If so, you’re not alone.   Leaf miners are out in force and are leaving their distinctive marks around Sherwood Park and Edmonton.

The good news about leaf miners is that their damage is cosmetic and, unless the plant is already very stressed, there’s no lasting harm.   The bad news is that, if you want to eradicate them completely, it’s a long and somewhat tedious road.

Leaf Miners 101

Rather than referring to a specific insect, leaf miner is a behavioral term that encompasses dozens of critters world wide.   The insect, whether a fly, moth, or beetle, lay their eggs on, or in, leaves so hatching larvae has a buffet of tasty chlorophyll waiting for them.

The larvae forms meandering tiny brown trails, resembling meandering rivers, as they chomp through the soft leafy insides. By the time we notice them, the trails have often already merged with each other and meandering lines have become brown splotches.   This usually represents the end of the larval stage and the worst the damage will get.

While leaf mining critters have about 3 life cycles per year, we usually don’t notice the damage until the 3rd and last (mid summer).   You can sometimes see the tiny larvae inside the leaf, but usually all you’ll see are the tunnels and the black “frass” (we all know the real name for it) they leave behind.

The insects overwinter by burrowing into the ground near the host plant, emerging the next spring to start all over again.

Although losing leaf tissue weakens plants by reducing their ability to photosynthesize, the damage is seldom fatal.   Keep it well-watered and fertilized and it will soldier on.   If your plant is badly affected, keep your eye out for other, hopefully more easily treatable, opportunistic pests or diseases that can strike a weakened host.

Edmonton-area birch trees are commonly afflicted with the Amber marked Leafminer (Profenusa thompsoni), which was introduced from Europe about 100 years ago.   The good news for birch lovers is that the city of Edmonton is world leader in biologically controlling this critter.

In the 90s the city introduced a parasitic wasp (stingless) which has established a population and helps keep birches in the Edmonton area clean.   It’s been such an effective control that the city hasn’t had to spray for over 15 years.

Controlling Them

There are no effective sprays for leaf mining critters while they’re inside the leaves.   All pesticides left on the market are contacts and won’t touch the larvae.

If you time it perfectly, you may be able to hit the critters as they’re emerging from the ground in the spring.   Apply horticultural oil as soon as the buds start unfurling (early spring), and apply weekly until June.   While you’ll need to persist for several years, this is usually an effective treatment.

For beets, tomatoes, and other smaller plants, I suggest opting for diversionary tactics over full out conflict.   Planting nasturtiums and columbine near the affected area will lure the critters from your prized vegetable and tomato plants.

Several species of wasps, including the species introduced by the city of Edmonton, feast on emerging leaf mining critters and can help keep populations in check.   Try to avoid the use of pesticides in order to protect any populations of predators you may have.