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The answer to the above question is that leaf miners usually are only a minor problem. According to Colorado State University Extension, “Although leaf miner injuries are conspicuous, most leaf miners produce injuries that have little, if any, effect on plant health.”
Usually, those pests’ natural enemies keep them in check, except when the enemies have been eliminated by the overuse of pesticides. So, if your idea of how to get rid of leaf miners is employing pesticides to provide yourself with a supposedly bug-free backyard, keep in mind that those sprays may leave you with more pests in the long run!
What Are Leaf Miners?
Leaf miners are the larvae of flies, moths, or beetles that lay their eggs on the undersides of leaves. Once the eggs on a particular leaf hatch 2 to 4 days later to begin the leaf miner life cycle, the larvae burrow into that leaf between its top and bottom layer.
Fortunately, they only chow down for 2 to 3 weeks before pupating inside the leaf or in the ground beneath the plant. Unfortunately, pupation only takes 10 to 25 days, so there can be several generations of these pests per year.
Three of the common types are vegetable leaf miners and serpentine leaf miners, larvae of Liriomyza sativae and trifolii, respectively, and spinach leaf miners, larvae of Pegomya hyoscyami. In all of these instances the leaf miner insect is a small fly, Liriomyza types having yellow markings and laying yellow eggs while the unmarked Pegomya type lays white ones.
Signs of Leaf Miner Damage
A stippling of small puncture wounds may appear on a leaf after a fly has laid its eggs on the underside, giving an alert gardener time to remove that leaf or the eggs before they hatch. If that natural pest control doesn’t happen, the leaf miners’ damage will become more pronounced once the larvae begin feeding.
As their name implies, serpentine leaf miners make what look like winding trails across a leaf, as do vegetable leaf miners. Damage from the spinach type, which actually isn’t confined to spinach and also can appear on chard, beet greens, and other vegetables, has a more blotchy appearance. Those tunnels or blotches eventually dry out and turn brown. So, leaf miner damage is unsightly but not usually dangerous to the plant unless a large part of it is affected.
How to Get Rid of Leaf Miners
Colorado State doesn’t recommend any controls for leaf miners. Many insecticides can’t reach them and may kill their natural enemies instead. To reduce leaf miner populations, remove and trash affected leaves and destroy any eggs you find. If you must spray, use organic insecticides such as spinosad or neem oil.
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Project Overview
Working Time: 15 minutes
Total Time: 20 minutes
Skill Level: Beginner
Estimated Cost: $12
Step 1: Decide which organic leaf miner control you want to use.
According to the University of Illinois Extension, “Neem oil can be applied to plants in the early spring and again every few weeks to repel flying (leaf miner) adults and keep them from laying eggs. Once the larvae are in the leaf, however, neem oil will not be effective.”
In other words, neem oil can fend off the adult form of miners but is not the best insecticide for exterminating them. If you miss the season for such preventative measures, T. Jude Boucher of the University of Connecticut Extension notes that spinosad can “move through the leaf cuticle to reach leaf miner larvae.” So it probably is the best vegetable garden insecticide for those pests.
Step 2: Choose the correct time of day to apply that insecticide.
Before applying even an organic leaf miner pesticide such as neem oil or spinosad, try other controls first. Although these two don’t adversely affect most beneficial insects, they can be toxic to bees when still wet on plants’ foliage. And any sprayed-on oil such as neem will smother beneficial insects as well as the bad ones.
Spinosad also may harm syrphid fly larvae and many of the caterpillars that produce butterflies. Therefore, only apply either leaf miner control in the evening when bees and many of the beneficials aren’t active, to allow that spray time to dry overnight.
Step 3: To prevent egg-laying, spray your plants with protective neem oil.
You can purchase neem oil either in a ready-to-spray formula or as a concentrate. For the latter, you must provide your own spray bottle and combine the amount of concentrate specified with the amount of water specified. Some brands may also recommend that you include a small amount of mild dish soap as an emulsifier, to help the oil and water blend better. Whether employing the ready-to-use spray or the blend as your leaf miner treatment, be careful to shake the bottle frequently and cover all surfaces of your plants’ foliage with the spray.
Step 4: To kill larvae already in the leaves, spray with spinosad.
As with neem oil, spinosad comes in both a ready-to-spray formula or concentrate. Again, if using the concentrate, you will need to follow the directions on the container when mixing it with water, shake your spray bottle frequently while spraying insecticide, and apply it thoroughly to all leaf surfaces. Keep in mind that neither neem nor spinosad kill pests instantly but usually stops them from feeding instead, so you may not see rapid results.
Step 5: Repeat the applications as often as recommended on the container.
For neem oil, you will need to repeat the application every 1 to 2 weeks. The amount of time you should wait before another application of spinosad leaf miner insecticide will vary with the type of crop you are spraying and ranges from 3 to 10 days. Due to the detrimental effect that even organic insecticides can have, you should never use them for longer than necessary.
How to Prevent Leaf Miners
Rather than having to consider how to get rid of leaf miners with an insecticide during the growing season, take time before that season begins to consider how you can prevent them. You may want to cultivate umbel-forming plants such as dill and fennel, which will attract parasitic wasps that prey on leaf miners.
It’s also a good idea to till your garden in autumn to kill any larvae attempting to overwinter in the soil. Also, avoid planting leaf-miner-attracting crops in the same area of your garden where they were the previous year. You can swath young plants with fine netting or row cover to prevent flies from laying eggs on them, but that only will work if there are no overwintering larvae in the soil beneath those covers. Finally, avoid excessive levels of nitrogen, which give leaf-miners a soft berth in plenty of tender new leaves!
