Leafminers leave telltale tracks

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

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

 By Debbie Arrington 

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