In a nutshell:
Maggot larvae of tiny true flies; common greenhouse pest of many plants; soft tissue feeder—lives and tunnels inside the leaf structure; leaves meandering silvery squiggles on the leaf surface; adult flies also “bleed” sap by making small cuts in the leaf surface to feed.
What are leaf miners?
Leaf miners, collectively, are the larvae of a group of small, true flies (of the order Diptera, like house flies and fruit flies). They are a common pest in temperate regions like Canada, especially in greenhouses. The larvae tunnel inside leaves (leaving behind “mine” tunnels, hence the name “leaf miner”). They’re relatively well-protected living inside the leaf itself, where they feed on its soft inner tissues.
What kinds of plants do they attack?
A huge variety of vegetables, fruits and ornamental plants can be host to a leaf miner infestation.
What do they look like?
Adult leafminer flies are very small (only a few millimeters in size), and they’re yellow and black in colour. The female fly cuts a tiny hole in the leaf of the host plant and lays an egg inside. The larva that hatches is a small (3 mm long), white or greenish-yellow, translucent maggot. It begins to feed on the leaf immediately, consuming the soft inner tissues while leaving the outer layers intact. When the larva is ready to pupate, it cuts an exit hole in the surface of the leaf and drops out onto the ground. It then either burrows into the soil or finds shelter in a crack or crevice and transforms into a small pupa, which can range from light yellow to black in colour. Sometimes pupae can be observed still hanging from the underside of leaves.
How do I know if my plants are under attack?
Larval tunneling creates the insect’s characteristic “leaf mines”; the larger the leaf miner, the wider the tunnel. The tunnels, themselves, appear as meandering whitish, silvery squiggles on the leaf surface. Sometimes leaf miner feces (frass) can be seen through the translucent tunnel walls as trails of tiny black spots.
The damaged caused by the tunnelling leaf miner can also cause the affected leaf to dry up and fall off. The loss of leaves causes cosmetic damage in ornamentals and reduces vegetable and fruit yield.
Adult leafminer flies also feed on host plant leaves by cutting tiny slits from which they drink the sap. These feeding spots can act as entry points for the transmission of bacterial and fungal disease and, in large numbers, the holes can damage the leaf.