While not usually threatening to plants, leafminer control is often necessary to manage the highly visible tunnels in leaves that can reduce crop value.
Adult leaf miners are tiny moths, sawflies, beetles, or flies. Most people never notice them. Though there are many different leaf miner species, they all have small caterpillars (moths) or maggots (everything else).
Leaf miners overwinter as pupae in the leaf litter and garden debris. They hatch in early spring when the weather warms. After mating, the adult female gets busy laying eggs on the bottom of leaves.
Yes, leaf miners are harmful. They affect the amount of chlorophyll in the leaf, which means it cannot photosynthesize as much energy for the plant. Badly infested leaves can drop off the plant entirely.