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Leafminers leave telltale tracks

Damage may not be pretty, but relax -- it's just cosmetic

Sacramento Digs Gardening
BY Debbie Arrington PUBLISHED AUG 24, 2020

This shishito pepper plant has evidence of leafminer damage on the leaves. (Photos: Kathy Morrison)

Some garden pests become a nuisance in hot weather. Their damage may not be pretty, but eliminating these little culprits can cause more harm than benefit.

Such is the case of leafminers. The larvae of little black flies, these worms cause serpentine trails on leaf surfaces as they tunnel (or mine) for food just below the surface.

The life cycle of this little pest is only two weeks, so there can seem to be sudden spikes in activity. As gardeners, we don’t notice the black flies – just the winding tracks on foliage left by the larvae.

According to UC Cooperative Extension master gardeners, leafminers attack a wide range of hosts, including cole crops (cabbage, kale, broccoli, etc.), cucurbits (squash, pumpkins, cucumbers, melons), tomatoes, peas, beans, asters, begonias, dahlias, impatiens, lilies, marigolds, petunias and verbena. This year especially, that list includes pepper plants.

Except for seedlings, this damage rarely harms the host plant. It’s cosmetic and really only is a problem for leaf crops such as cabbage, kale, chard and spinach. Protect those seedlings with crop covers. (They probably will appreciate a little shade, too.)

Treatment generally is not required for leafminer damage.

As for other crops, trim off and dispose of invested leaves. Pesticides aren’t particularly effective against leafminers; spraying has to be perfectly timed to actually work. And any spraying will kill the beneficial insects that feed on the leafminer flies and larvae, so it’s doubly bad.

For ornamental plants, systemic pesticides can dissuade the leafminer from munching (but it also makes the plant inedible for people).

Neem oil often is recommended as a way to generally treat any pest infestation. But applied in the heat of August, neem oil will fry the foliage, not save the plant.

Instead, here’s the advice from the UC integrated pest management website:

“Leafminers rarely require treatment in gardens. Provide proper care, especially irrigation to keep plants vigorous. Clip off and remove older infested leaves.

"Plant resistant species or varieties. Small seedlings can be protected by protective cloth. On plants such as cole crops, lettuce, and spinach, clip off and remove older infested leaves.

"Leafminers are often kept under good control by natural parasites. Insecticides are not very effective for leafminer control.”

So, if you see squiggles on leaves, reach for your scissors first. More on leafminers: http://ipm.ucanr.edu/PMG/GARDEN/VEGES/PESTS/vegleafminers.html

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UC Cooperative Extension UCCE master gardeners gardening Sacramento pests vegetable gardening ornamentals UC IPM

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Garden Checklist for week of Nov. 3

November still offers good weather for fall planting:

* If you haven't already, it's time to clean up the remains of summer. Pull faded annuals and vegetables. Prune dead or broken branches from trees.

* Now is the best time to plant most trees and shrubs. This gives them plenty of time for root development before spring growth. They also benefit from fall and winter rains.

* Set out cool-weather annuals such as pansies and snapdragons.

* Lettuce, cabbage and broccoli also can be planted now.

* Plant garlic and onions.

* Keep planting bulbs to spread out your spring bloom. Some possible suggestions: daffodils, crocuses, hyacinths, tulips, anemones and scillas.

* This is also a good time to seed wildflowers and plant such spring bloomers as sweet pea, sweet alyssum and bachelor buttons.

* Rake and compost leaves, but dispose of any diseased plant material. For example, if peach and nectarine trees showed signs of leaf curl this year, clean up under trees and dispose of those leaves instead of composting.

* Save dry stalks and seedpods from poppies and coneflowers for fall bouquets and holiday decorating.

* For holiday blooms indoors, plant paperwhite narcissus bulbs now. Fill a shallow bowl or dish with 2 inches of rocks or pebbles. Place bulbs in the dish with the root end nestled in the rocks. Add water until it just touches the bottom of the bulbs. Place the dish in a sunny window. Add water as needed.

* Give your azaleas, gardenias and camellias a boost with chelated iron.

* For larger blooms, pinch off some camellia buds.

* Prune non-flowering trees and shrubs while dormant.

* To help prevent leaf curl, apply a copper fungicide spray to peach and nectarine trees after they lose their leaves this month. Leaf curl, which shows up in the spring, is caused by a fungus that winters as spores on the limbs and around the tree in fallen leaves. Sprays are most effective now.

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