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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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Get outside early in the morning while temperatures are still cool – and get to work!

* Plant, plant, plant! It’s prime planting season in the Sacramento area. Time to set out those tomato transplants along with peppers and eggplants. Pinch off any flowers on new transplants to make them concentrate on establishing roots instead of setting premature fruit.

* Direct-seed melons, cucumbers, summer squash, corn, radishes, pumpkins and annual herbs such as basil.

* Harvest cabbage, lettuce, peas and green onions.

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* Plant dahlia tubers.

* Transplant petunias, marigolds and perennial flowers such as astilbe, columbine, coneflowers, coreopsis, dahlias, rudbeckia and verbena.

* Keep an eye out for slugs, snails, earwigs and aphids that want to dine on tender new growth.

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* Are birds picking your fruit off trees before it’s ripe? Try hanging strips of aluminum foil on tree branches. The shiny, dangling strips help deter birds from making themselves at home.

* As spring-flowering shrubs finish blooming, give them a little pruning to shape them, removing old and dead wood. Lightly trim azaleas, fuchsias and marguerites for bushier plants.

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