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Leafminers leave telltale tracks

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

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.)

Leaves with leafminer damage
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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Flowers in My Back Yard Series

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Sept. 8: A must for fall gardens, asters are little powerhouses

Sept. 1: Yarrow an easy-care, sturdy flowering plant

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June 16: Daylilies are perfect for water-wise gardens (and a lot more)

June 9: Grow coneflowers for pollinators -- and yourself

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May 26: Zinnias are the summer flowers every garden needs

May 19: Plant dahlias now for late-summer flower power

May 12: Know your coreopsis from your bidens

May 5: Mums the word on Mother's Day weekend

April 28: Majestic Matilija poppy is worth a look

April 21: Celebrate roses, America's favorite flower

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March 10: Keep camellias happy for years to come

March 3: Fruit tree blossoms are a fleeting joy

Feb. 27: Are your roses looking rusty?

Feb. 24: Treasure spring daffodils now and for years to come

Feb. 17: How and why to grow wildflowers

Feb. 10: Let's talk Valentine's Day roses

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Garden checklist for week of Sept. 13

Now is the time to plant for fall. The warm soil will get these veggies off to a fast start.

* Keep harvesting tomatoes, peppers, squash, melons and eggplant.

* Compost annuals and vegetable crops that have finished producing.

* Cultivate and add compost to the soil to replenish its nutrients for fall and winter vegetables and flowers.

* Fertilize deciduous fruit trees. Make sure to deep water before and after feeding.

* Plant onions, lettuce, peas, radishes, turnips, beets, carrots, bok choy, spinach and potatoes directly into the vegetable beds.

* Transplant cabbage, broccoli, kale, Brussels sprouts and cauliflower as well as lettuce seedlings.

* Sow seeds of California poppies, clarkia and African daisies.

* Transplant cool-weather annuals such as pansies, violas, fairy primroses, calendulas, stocks and snapdragons.

* Divide and replant bulbs, rhizomes and perennials.

* Dig up and divide daylilies as they complete their bloom cycle.

* Divide and transplant peonies that have become overcrowded. Replant with “eyes” about an inch below the soil surface.

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