Leafminer
Various (Agromyzidae, Gracillariidae, Tenthredinidae)
41 host plants
Last updated
Various leafminer species create pale, winding or blotchy tunnels within leaf tissue of alder, willow, poplar, and other host plants starting in spring. The miners are small larvae that feed between leaf surfaces, leaving papery trails as they consume leaf tissue. You identify the problem by the distinctive mining patterns visible on affected leaves; heavy infestations can cause significant leaf yellowing and drop.
Monitor host plant foliage starting in May for early mining appearance. Remove and destroy heavily mined leaves by hand to reduce overwintering populations. For valuable plants, apply horticultural oil in late winter to target pupae and overwintering stages. Once mining damage appears, spinosad or neem oil applied to undersides targets young larvae.
Quick Reference
Monitoring & Action
Monitor leaf undersides for adult flies in April-May (yellow sticky cards catch some). Observe new foliage for first signs of blotches or trails (May-June). By the time mines are visible, the larva is protected inside leaf tissue and beyond reach of sprays. Hold heavily mined leaf to bright light to look for tiny cream-colored larvae or small dark pupae - this confirms active infestation vs. completed cycle.
For established landscape plants: no action threshold. Damage is cosmetic and plant remains healthy. For high-value ornamentals (formal hedges, specimen plants) or nursery stock: 90% of leafminer damage does not warrant treatment on mature plants. Treatment justified only if: 1) Young stressed plants (first year, poor soil), 2) Formal ornamental where cosmetic damage is unacceptable, 3) Nursery production where quality matters. Natural enemies (parasitoid wasps) typically control populations to tolerable levels without intervention.
The beet or spinach leafminer is the larva of a small (1⁄4") gray fly with black hairs. Eggs are laid on the leaves of beets, chard, spinach, and weeds including lambsquarters. The emerging maggots mine between the upper and lower surfaces of the leaf, forming narrow mines which later enlarge into pale blotches. Damaged leaves are distorted. The white larvae are about 1⁄4" long when mature. They emerge from the leaves and pupate in the soil.
Cultural Controls
- Sanitation pruning: In early fall, prune off and destroy heavily mined leaves (do not compost). This removes pupating larvae before overwintering, reducing next year's population. Labor-intensive; practical only on small plants.
- Accept damage on established plants. Parasitoid wasps (native beneficial insects) lay eggs inside mined leaves; their larvae consume leafminer larvae. Wet climate and diverse flora support robust parasitoid populations. Doing nothing is not laziness; it is letting nature do its work.