Maggots are eating my chard!

Poking around the garden last week, I noticed a few spots on the leaves of chard and beets where the leaf looked pale, burned or papery. A few days later the damage had spread, and some of the leaves were dying. So I started googling and consulted fellow Niles gardeners to see if anyone knew what was happening. The answer: Leafminers, miniscule maggots (fly larvae) from tiny flies that tunnel into leaves and consume them from within. They can infest both veggies and flowers.

“These pests can do a lot of eating sometimes before we even notice them,” John, from the Alameda County Master Gardener Help Desk, told me in an email.

He gave me a crash course on this sneaky pest:

“Leafminers are very small (1.8mm) yellow and black flies. The females insert their eggs between the leaf tissues of chard, beets, tomatoes, etc. and as the eggs develop they eventually eat the whole leaf.  Since leafminers feed on a large variety of plants it’s good to keep the area around your produce relatively free of unwanted plants, such as weeds, to reduce the attraction of your garden.

“I tend to focus on keeping the plants well cared for and watered, occasionally water blasting the leaves and clipping off the older infested leaves and disposing of them deep in my compost bin or the city green bin.  Some other options include covering new plants with a protective cloth.”  

The little varmints drop down to the soil to pupate, so clipping off the affected leaves as soon as  you notice them can interrupt this cycle. (Also look for these little white cylinders on the backs of leaves; those are the eggs, usually next to a trail where hungry larvae have set out. Be sure to dispose of this part of the leaf, too.)

A cluster of leafminer eggs on the underside of a leaf. The hatched larvae tunnel into the leaf and eat it from within, creating squiggly trails and broad patches of damage, before dropping to the ground to pupate. (Photo by Glennda Chui)

Leafminers can produce five to 10 generations per year, according to the UC Integrated Pest Management website. The miners rarely kill the plant, but if you want to eat the leaves, that’s small consolation.

Some folks on the Internet advise spraying affected leaves with a solution of water, vegetable oil and natural soap to smother the larvae; others suggest trying to squish the larvae within the leaves. (I can’t see the larvae so this seems impractical.) Still others say external chemical treatments won’t work because the larvae are safely out of reach. If you find anything that works, I would love to hear about it.

By the way, if you ever want help with a gardening issue, here’s contact info for the Master Gardener Help Desk:

Master Gardeners Help Desk hours:  M, W, Th, 10am to 1pm

(510) 670-5645; acmg@ucanr.edu

Website: http://acmg.ucanr.edu/

Click to subscribe to the monthly ACMG newsletter.

The UC Master Gardeners are volunteers trained by the University of California Cooperative Extension who provide free advice and information to their communities, all based on solid research.  I’m so grateful that we can learn from these experts in our midst!

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