Leafminers are members of the order Lepidoptera, like butterflies and moths. They undergo complete metamorphosis (holometabolic insects) and pass through the egg, larva and pupa stages.
Eggs: Greenish yellow, 3 mm long.
Larvae: Black head and a light to dark green body tinged with red, 5 mm long.
Pupae: Leaf green with a brown head, 3 to 4 mm long.
Adults: Small moths (2.5 mm long, with an 8 mm wingspan), grey with brown wingspots.
Leafminers produce a single generation each year. They overwinter as larvae, inside arborvitae scales.
In early spring (mid-May), the larvae wake up and begin feeding, tunnelling into twigs. In late May, they build cocoons inside scales or under the foliage, depending on the species, and pupate.
The adult moths emerge from the cocoons and are active from mid-June to mid-July. After mating, the females lay their eggs (about 25 eggs per female) around the edges or tips of the tender new leaves. As soon as they hatch, about 20 days later, the tiny caterpillars start mining into the leaves.
In fall, the larvae stop feeding and overwinter in the tunnels.