Golden Gate Garden

Battling a Swiss Chard Pest

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Every year, the leaf miners ruin my Swiss chard all summer. Maggots in the leaves, with their accompanying frass (bug doo) render it rather unappetizing. In past years, I have sprayed with summer oil every week, or as often as I can remember to do it, but the improvement has been only slight.

I have been reading about a new product, called Spinosad, and have finally tried it on my Swiss chard. About a week and a half after the first spraying, there seem to be fewer new injuries. I also sprayed spinach that was being damaged, with similar results. Tomorrow, I will clean up the plants and spray it again.

Spinosad (also sold as Bulls-Eye), is made by fermenting the bacterium Saccharopolyspora spinosa. Two of the metabolites of this bacterium (that is, products of its metabolism) are highly toxic to a number of insects. It is said to stop susceptible insects from feeding in one hour, and remain toxic to newly arriving insects for the next week or two. It is also said to spare lady beetles, lacewings, and minute pirate bugs, all creatures that eat pest insects in our gardens. It breaks down in sunlight and does not persist in soil. The amount of time to wait after spraying before you harvest varies, but for Swiss chard and other leafy greens, it is one day. It is OMRI, meaining approved for use by organic farmers.

At this point, I don’t know every detail about this material, but if the leafminers desist from mining in my Swiss chard leaves, I will find out the ones I don’t know yet and report them. It is also said to kill thrips, caterpillars, sawflies, and a number of other pests. (A critter called a rose slug is a sawfly larva. I see it on San Francisco roses often. Summer oil is relatively effective against it, but for serious infestations, Spinosad may be just the ticket.)

By the way, the leafminer in the chard and spinach is the larva of a fly (therefore it’s a maggot) that eats out the inside of leaves, causing ugly blotches.The fly lays eggs on the leaves of chard and spinach; the larvae hatch out and enter the leaves. When they have eaten their fill, they drop to the ground to pupate. When they emerge from the pupae, the flies breed and lay eggs again. This goes on in the warmer part of the year here in San Francisco, from late March until mid-October. (Chard and spinach growing October to March are safe, but you have to start them earlier than mid-October for good winter production.)

The fact that the pupae live in the soil means that beneficial nematodes watered into the soil might help as well. I am thinking of getting some this year. These microscopic creatures that you can purchase are only effective when the soil is warm, so I will wait a few more weeks before adding them.

17 responses to “Battling a Swiss Chard Pest”

  1. dee

    Boric acid is very effective against these pests . From http://pests.in

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  2. pam

    I don’t have any info about boric acid and leafminers. Boric acid is effective against ants. The link provided in the previous post is a link to a site dealing with indoor pests, not garden pests. I don’t recommend use of boric acid for leafminers in the garden.

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  3. charlene oneil

    where to buy bulls-eye in Bay area?

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  4. charlene

    I have a question, as I am a new gardener. Is it OK to eat spinach that has had the leafminer larvae on it? I have just painstakingly washed, leaf by leaf, a small harvest of spinach, brushing the larvae off each until I am sure they’re gone. The last thing I want is to contract a parasite. I don’t know what is safe or standard practice in terms of eating from an imperfect garden. Advice??

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  5. pam

    Charlene,
    You can’t get rid of leafminers by washing the leaves. The larvae live inside the leaf, between the top and bottome “skins” of the leaf. They lay eggs at first, on the outside of the leaf, and you can brush these off, but then the eggs hatch and enter the leaves.
    The leafminer larvae wouldn’t survive being eaten by you. They are totally incapable of surviving inside you and do not parasitize anything but spinach, beets, and Swiss chard. Most people wouldn’t want to eat them due to the ugh factor, but they would just be, as they say, extra protein, nutritionally speaking.
    In order to avoid eating them, most people would cut away the part of the spinach or chard leaves that had damage, problem being, of course, that if spinach leaves aren’t very big, you might not have much left to eat.
    There is more on this pest in later posts, including ideas for controlling it. Do a search for “chard” to find them. There are photographs.
    Yes, gardeners often, within reason, wash off the pests and eat the rest, but this one is a hard one to deal with once it is in there.
    If you don’t mind, I would like to consider your question for a column that I write for the SF Chronicle (and sfgate.com).

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  6. chard man

    Leaf miners are no problem. Wash the eggs of the leaves every few days. If you somehow miss one and they get into the leaf, cut the mined part right out of the leaf.
    I have a 100 percent success rate this way.

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  7. Teri in Minneapolis

    Sticky paper also helps by trapping the flies and interrupting the reproduction cycle. I remove the leaves that have been mined or just the affected parts on larger leaves. Early intervention is important–if you catch it early the odds of success are better. Bulls Eye sounds fishy to me! Neem is supposed to help but I had no luck with that.

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  8. Richard

    This information has saved me a trip to the nursery and I thank you for that. I also have romaine and red leaf lettuce in the garden and currently haven’t seen any problem yet. Do they eat lettuce too ? Thanks for any help.

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  9. Keith

    I had unseen bugs that skeletonized the leaves of my Swiss Chard this year. The “frass” mentioned in the article was present at the end (before my plants died!) I think most of the larvae were gone by the time I tried harvesting, and even the stems seemed damaged. This was the first year I had this problem, as Swiss Chard is pretty reliable normally. Also, in answer to the previous post, none of my lettuce was affected – just the Swiss Chard. I may try some rotenone next season as it breaks down fairly quickly before harvesting.

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  10. Pam Peirce

    Hi Keith,
    I am perplexed by your “unseen bugs.” By skeletonized,do you mean they ate all the leaf except the midrib and large veins? Can I ask where you are gardening? Are you in the SF Bay Area? Have you looked at your chard at night, with a flashlight to check for nocturnal pests? I’d like to figure out what the pest is before you choose a pesticide.

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  11. Lynette

    I too have had this problem with my beets and chard for about two years! I read an article in the Chronicle regarding this pest, and have been picking the leaves faithfully but nothing has made it stop and it’s now almost December. I have even tryed food grade DE like I use on my brasicas! Next step will be the Bulls-Eye as we love our greens!

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  12. Larry McMahan

    This is great news to me! My swiss chard has been ravaged by these pests this years. It just happens that I have been spraying my Cherry tree for Spotted Wing Dorsophila (Cherry Vinegar Fly) with Spinosad. I will just start spraying any chard which I plant also. I was wondering what I was going to do other than wait a year or two for the leafminers to leave. Great info!

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  13. Larry

    Is a sprinkling of Diatimatious Earth of any use? What about 2 cups water & 2 teaspoons of dishsoap (w/o Bleach), & maybe the addition of diced hot peppers (~1/2 cup) &/or garlic (~5 or so cloves crushed)? I believe you put the dishsoap in after you have let the other ingredients brew for 24 hours & strained the pulps out. It is supposed to work with spittlebug, & I heard similar remedies work against ants & aphids. Ive also heard/read 10 parts water to 1 part vinegar works against aphids, I think. I am returning to gardening just recently, after not having done much of it, except weeding, since I was less than 10 years old. I am surprised at how few of the details I am now, or then was barely aware of, because my mother or grandmothers must have been attending to it back then. I know that we had pests, & I know they did things, many with simple household materials & a spray bottle, but I apparently wasnt pay as close attention as my adult self wanted to believe a few months ago.

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  14. M Iasimone

    Frustrated …leafminer destruction! Try so hard to stay on top of gardening chores to no avail with these leaf miners. Besides turning over the ground with a tiller are there any products that I should/could introduce into the earth to help destroy the pupae that may still be present in said ground?
    I also have been using manure ( chicken manure being my preferred choice) could this be a contributing factor for flies/ pupae/larvae? Any suggestions?

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  15. Pam Peirce

    Use of manure will make no difference in battling chard leaf miner (or the other flies whose larvae prey on our different crops). These are not filth flies. They are flies that depend only on particular plants to feed their larvae. Beet and chard leafminner feeds only on these plants. There is a root maggot species that feeds only on roots of the cabbage family plants, another on onions, and yet another on carrot roots.
    To keep the pupae out of ones soil, one has to keep them from forming. Start by looking at leaf undersides and brushing off any small white eggs you see. Those are the leafminer eggs. If any hatch and start feeding in a leaf, cut out the damage right away, or remove the leaf and get it out of the garden. Turning the soil after you remove the chard can help if you think some pupae have fallen in it.
    Try growing the chard only when night are chilly. I have been starting mine indoors, in mid or late summer and planting them out in September. I use row cover, with its edges tucked firmly into the soil all around so no flies can get in, until weather turns cold (a couple of nights in the 40s), then uncover the plants and harvest all winter. But if climate change continues, this may not work.)
    I suspect that nothing you spray will help a lot, since it can’t kill the larvae once they are inside the leaf, but if you have a lot of plants, it might help to spray Spinosad regularly to get the eggs. (Forget Rotenone, which has been linked to Parkinson-like symptoms.)

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  16. Ann Reynolds

    I also have something eating my chard. And my rhubarb and my Hostas! Mostly causing holes in the middle of the leaves. On the rhubarb they ate from the edge of a new leaf. I just planted the rhubarb so the leaf was very small. I just cleaned up the chard and threw away about 2 meals worth of leaves. So sad. We did spray with Sevin for the hostas and rhubarb, but wasn’t sure about the chard. I think the damage has been stopped for the ones sprayed. Would diatomaceous earth help?

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  17. Pam Peirce

    Please do not use Sevin as a first-line tactic against pests. In the first place, you have not identified the pest, so you don’t know if the chemical is effective against it. In the second place, Sevin is highly toxic to honeybees and to beneficial insects that would otherwise be helping you control insect pests. It also kills earthworms if it gets into the soil, and cats are quite sensitive to exposure.
    Your first job is to find out what is eating your plants. (I’m sorry to be a few months in replying, since the pests of spring may not be the pests of fall.) In spring, earwigs may be the culprit. But at any time of year, it is likely to be snails and/or slugs (which are not listed on the Sevin label).
    The best way to find out what is chewing holes in leaves is to venture out at night, a couple of hours after dark. I use a headlamp, so I can have free hands. If you find snails, preferably kill them on the spot, ad it you do this for 3 nights, you will probably have none for some months after. You can also apply Sluggo iron phosphate bait. If you see earwigs on your plants, use Sluggo Plus. This and much more information on managing common garden pests wisely is in my book Golden Gate Gardening.

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