Leaf miners on Plants

Symptoms

The first sign you are likely to notice is pale or white blotches on the leaves. These blotches appear where the larvae have fed. On an affected plant the pale blotches can be numerous and round.

Look more closely and you may see a linear series of small round white dots on a leaf. Those dots mark the punctures where adult flies laid eggs. Adult flies puncture leaves while laying eggs, and they feed on leaf sap too. The blotches mark feeding, while the dots mark egg-laying punctures.

The larvae feed inside the leaf tissue, and the mines they leave disfigure the foliage. When an infestation is severe, the damage can make plants unmarketable. The pest has multiple generations per year, and populations can build quickly. These signs can appear on many hosts, including tomato, peas, beans, begonia, dahlia, impatiens and petunia. Pale blotches, white dots and disfigured foliage are the three signs listed for this problem.

Causes

Leaf miners are the larvae of flies, for example Liriomyza species. The larvae feed and tunnel within the leaf tissue. Fly larvae are the cause named for the pale blotches on the leaves.

Adult flies feed on leaf sap and puncture the leaves while laying eggs. Those punctures are the small white dots you can see on the leaf surface. Egg-laying punctures and feeding by the adults add to the damage the larvae do.

The flies have multiple generations per year, which allows populations to build quickly. One example is the allium leafminer, often shortened to ALM. It has two generations per year. It overwinters as a pupa in leaf tissue or in adjacent soil.

Several host plants are affected, among them tomato, pea, bean, aster, begonia, dahlia and impatiens. Citrus, kumquat and calamondin are also on the host list. Caloptilia azaleella is among the leafminer pests of azalea and rhododendron.

Diagnosis

Start by looking at the pattern of the damage on the leaves. Leaf miner damage shows as pale or white blotches where larvae have fed. The sign that points to leaf miners is a linear series of small round white dots, which mark egg-laying punctures. Look for those dots on the leaves next to the blotches.

The larvae feed within the leaf tissue, and the pale blotches mark where they have fed. Adult flies puncture leaves while laying eggs, so the dots are made by the adults rather than the larvae. Both marks together separate this problem from a general care fault, because both come from the fly's life cycle.

Next, check the plant against the host list, which includes tomato, pea, bean, aster, begonia, dahlia, impatiens, lily, marigold, petunia and verbena. Citrus, kumquat, calamondin, azalea, rhododendron and chrysanthemum are also listed hosts. A severe case is easy to confirm, since the mines disfigure the foliage and can make plants unmarketable. The allium leafminer overwinters as a pupa in leaf tissue or adjacent soil.

Treatments

The cultural measures are listed first. Steam treat planting beds before you replant. Pasteurize growing media after you remove infested crops. Keep growing areas, and the sites that border your crops, free of weeds. Dispose of plant debris promptly, in covered containers.

Use row covers over susceptible crops and leave them in place until adult fly emergence ends. Adult flies puncture leaves to lay eggs, and the covers stay on until the adults have finished emerging. Scout the crop closely before you install covers, to confirm that leafminers are not already active. Steam treating, pasteurizing, weed control and row covers are cultural methods, not insecticides.

Among the organic options, certain formulations of the botanical insecticide azadirachtin can be applied. Another organic option is two consecutive sprays of spinosad mixed with a low-concentration soap solution. That spinosad mixture includes soap at a low concentration, and the product label covers every other detail of the mix.

The chemical options are foliar applications of dinotefuran, cyantraniliprole or spinetoram. A spreader-sticker adjuvant is listed under the chemical methods, as part of an insecticide application rather than a treatment of its own. The product label is the legal instruction, and you must follow it for the crop and site being treated. This page gives no dose, rate, dilution or spray interval.

Prevention

Weed-free conditions are part of prevention, so keep growing areas and bordering sites free of weeds. Clean up after infested crops by pasteurizing growing media, and steam treat planting beds before replanting. Dispose of plant debris promptly, in covered containers.

Physical barriers are another condition to set up: row covers over susceptible crops, left in place until adult fly emergence ends. Scout closely first to confirm leafminers are not already active.

The pest has multiple generations per year, and populations can build quickly. The allium leafminer overwinters as a pupa in leaf tissue or adjacent soil. Prevention here means conditions in the bed, the growing medium and the area around the crop. Weeds, debris and used growing media are the three conditions the cultural methods address.

Plants most often affected

Many kinds of plants can be hosts for leaf miners. Hosts include tomato, pea, bean, aster, begonia, dahlia, impatiens, lily, marigold, petunia, verbena and chrysanthemum. Azalea, rhododendron, citrus, kumquat and calamondin are listed as well. The leafminer pests of azalea and rhododendron include Caloptilia azaleella.

• American Basswood
• Arborvitae
• Common Lilac
• Littleleaf Linden
• New England Aster
• Oxeye Daisy