Key takeaways
- Leafminers tunnel through the mesophyll — the layer between the upper and lower leaf surfaces — leaving signature squiggly mines.
- They’re mostly members of two orders: Lepidoptera (moths) and Diptera (flies). This page focuses on the Dipteran (fly) leafminers.
- Damage is usually cosmetic, not life-threatening — but on retail-grade plants like chrysanthemums it destroys salability fast.
- The fly lays an egg in a puncture wound; the maggot tunnels, frass is forced out of the mine’s entrance, and pupation happens in the soil — meaning soil-pest controls double as leafminer controls.
- Preventive go-to controls: parasitic wasps (deployed early), yellow sticky traps for monitoring, and beneficial nematodes that target the soil-dwelling pupae.
Leafminers are tiny insects whose larvae tunnel through the mesophyll of leaves, leaving distinctive squiggly mines. They mostly belong to two orders — Lepidoptera (moths and butterflies) and Diptera (flies) — and this page focuses on the Dipteran leafminers most relevant to greenhouse growers.
Background
Leafminers are pests that tunnel through the mesophyll of leaves, creating distinctive and unsightly squiggly patterns. Collectively they are mostly members of two insect orders: Lepidoptera (moths and butterflies) and Diptera (flies). The latter is the focus of this page — caterpillar leafminers are covered on the caterpillars page.
There are roughly 98,000 members of the order Diptera, and many are pests in some way, but only a handful operate as leafminers. In the United States they aren’t as common as aphids or caterpillars, but they still deserve attention. Two decades ago they were a more serious problem than they are today, but they remain a concern for growers — especially in floral and ornamental crops.
How They Become a Problem
Leafminers can attack outdoor and indoor crops of many kinds, but the damage isn’t usually life-threatening. Aesthetically, though, these flies can destroy the value of a crop. Larger plants, shrubs and trees can be infested year after year without noticeable growth setbacks — unless the infestation is serious enough to hinder photosynthesis.
Cosmetic issues aren’t always a concern unless the plant is to be sold outright. Grower concerns usually center on leafminer infestations of smaller specimens grown for their appearance rather than their fruits — chrysanthemums are a textbook example. On mums and other flowering ornamentals, leafminers become a serious commercial problem.
On retail-grade ornamentals, leafminer damage is purely cosmetic — and it’s catastrophic. Once the mine is visible, the leaf is unsalable.
A Serious Pest
The infestation process begins as a female fly lands on a leaf and begins to feed by puncturing the surface with her sucking mouthparts. The activity creates a tiny white spot that, by itself, isn’t of great concern — though heavy feeding alone can make foliage unsightly. The real damage begins when the female lays an egg in the puncture wound.
The egg hatches and the resulting larva (maggot) tunnels through the leaf, eating the mesophyll cells and creating a void, tunnel or mine as it goes. The outer leaf surfaces remain intact, but with no cells between them the feeding becomes very visible and ugly. As the maggot grows the mines get larger and more conspicuous, and the maggot’s frass is forced out of the mine entrance — creating yet another unsightly mess (made worse when the maggot has been parasitized).
When the maggot is ready to pupate via complete metamorphosis, it usually drops to the ground. This is the critical control window: on the soil surface, leafminer pupae can be killed with one or more soil-pest controls. When the cycle completes, the pupae give forth a new generation of adult flies ready to mate and lay eggs again.
For the Scout
You probably won’t be bothered by large numbers of leafminers and, unless they’re a known problem pest in your area, we don’t recommend special treatments. Become visually familiar with them — difficult, since they’re unremarkable two-to-three millimeter flies with a yellow spot on each side of the thorax.
Using yellow sticky traps during a regular scouting regimen is the main step. Beyond that, watch for the tell-tale white puncture spots and early mines on susceptible crops like chrysanthemums, vegetables and seedlings. Catch the first mines and you can intervene before secondary generations build.
What the mines tell you
Fresh, narrow mines mean early instars — the larva is small and the leaf can still recover cosmetically if you control quickly. Wider, longer mines with frass trails mean a late instar near pupation — the leaf is lost, but a soil-targeted control program now will hit the next generation.
Some Solutions for Leafminer Control
There are few biological control agents specifically targeting Dipteran leafminers, but the parasitic wasps that are available do a very good job. Use them preventively, or at least at the very first sign of these flies.
Other biocontrol agents intended for soil pests — especially beneficial nematodes — also help with leafminers while being deployed primarily against fungus gnats and other soil-stage pests. The pupae have to drop to the soil at some point in the cycle, so soil-applied controls are a free hit on the next generation.
Chemical controls can be effective, but expect difficulty when treating leaf-protected larvae and soil-protected pupae. Pesticides offering systemic protection may work, but rotate active ingredients to prevent resistance build-up. Combine biocontrol agents with sticky-trap monitoring and you’ll keep leafminer pressure manageable.
Leafminer pupae live in the soil. Any soil-pest program you already run for fungus gnats is doing double duty against the next leafminer generation.
For over 30 years, Green Methods has been a leader in the production and application of beneficial organisms used in biological pest control and integrated pest management programs.