How to Combat Leaf Miners

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When you see wiggly, squiggly lines in the leaves of your trees, shrubs, ornamental plants, and vegetables, you can be sure leaf miners are at work. If you leave them to their ways, they can end up wreaking havoc on your plants and causing significant damage. To prevent this, you need to take an active role in getting rid of them.

Leaf miners are the larva of several different species of flies, sawflies, moths, and beetles.   They are found all over the world and the damage they do is very distinctive.  

These insects attack many different kinds of plants, but the damage, while unsightly, is only a big problem on vegetable crops.

By knowing how to identify them, you will be able to spot them and control them. It’s impossible to keep these pests out of your garden, but with good scouting and effective deterrents, the damage can be minimized.

Identification

These tiny, worm-like pests are about 1/8th inch long and can be identified by the damage they do.  They feed between the upper and lower surfaces of leaves, creating tunnels, trails, and blotches of dead tissue. These trails can be white, brown, or yellow.  When they mature, they develop into small black flies or moths that we rarely notice.

Photo credits to Alan Vernon

Leaf miners are attracted to lots of different plants.  Here are some of their favorites.

  • Trees – birch, alder, aspen, cottonwood, elm, magnolia, locust, oak
  • Evergreens (where they are called needle miners) – pine, arborvitae, juniper
  • Broadleaf Evergreens –  boxwood, holly
  • Shrubs – azalea, lilac
  • Flowers – columbine, butterfly weed, chrysanthemum, delphinium, impatiens, lantana, verbena, bougainvillea, and even water lilies
  • Food crops –  apple and citrus trees, blackberries, beets, tomatoes, garlic, onion, beans, peppers, spinach, chard, lettuce, cabbage, celery

On ornamental plants, the damage is mainly cosmetic, and controls are usually not necessary.  It takes a very heavy infestation to kill an ornamental.  But on leafy food crops, they can make the plant inedible because of the excrement they leave in the trails.  Remember, these pests eat the foliage – not the roots or fruits – so crops like cucumbers, beets, and tomatoes can still be eaten even if you find trails on their leaves.

Photo credits to Smabs Sputzer

Life Cycle

Adult females lay tiny white eggs, usually on the underside of foliage, but some species lay eggs inside the leaf itself.  Each female can lay up to 250 eggs.  In about a week, larvae hatch and begin feeding.  The first generation occurs in spring, about the time that lilacs are almost finished blooming.

The larvae eat the juicy middle of the leaves and make tunnels or blotches as they munch their way through the insides.   In 1 to 3 weeks, the larvae are ready to pupate, and they bite a slit in the edge, wiggle out, and drop to the ground to burrow into it.  Adults emerge in another 1 to 3 weeks and the cycle begins again.  There can be several generations in a season, and the pupae can overwinter in the soil.

Prevention and Control Methods

It’s impossible to keep all leaf miners out of your garden, but a few simple actions can help keep populations at a manageable rate.  As we already mentioned, protecting vegetables is important.  Also, young trees may need protection in their first few years.

Keep up with scouting.  As we gardeners know, being out in our gardens allows us to enjoy the beauty of our hard work, but also alerts us to insect problems and signs of disease.

Cultural Controls

Keeping your plants healthy is always the first line of defense against any pest problem.  Strong, healthy plants can withstand attacks from pests better than stressed and sickly plants.

Avoid over-fertilizing and keep pruning to a minimum during the seasons when these insects are active.  These pests prefer the tender new growth that fertilization and pruning often cause a plant to produce.

Row covers can be installed to protect susceptible crops.  Be sure the edges of the cover are secure, so the adults can’t fly underneath.

You can interrupt the life cycle in a couple of ways.  The larva can be squished while they are in the leaves – just crush them between your fingers.  This is the most effective killing method.  Or infested leaves can simply be removed and destroyed.  Tilling your garden will destroy any overwintering pupae.

Adults can be controlled by using sticky traps. They are attracted to blue and yellow.  Hanging a few traps around the plants they like will attract the adults.  They then get stuck on the trap and cannot fly away to mate or lay eggs.

Some gardeners like to use trap crops like columbine, lambsquarter, and velvetleaf hoping that the insects will use these plants and leave their vegetables alone.  However, this can backfire and you can end up increasing the population in your garden.  You can experiment and see if this is an effective control method for your garden.

Biological Controls

Beneficial insects like parasitic wasps that feed on the larva and nematodes that feed on the pupae probably already exist in your garden.  If you want to increase the populations of beneficial insects, you buy them at some garden centers or purchased them online. 

If you are using biological controls, remember not to use pesticides.  Insecticides will kill the beneficial insects too.

Chemical Controls

Insecticides will not reach the larva once they start feeding. Eggs are invulnerable and adults just fly away.

The best time to use an insecticide is when the tiny larvae first hatch out – that happens when common lilacs have almost finished blooming and when Vanhoutte spirea is in full bloom.  An insecticide can be applied just before the eggs hatch or a systemic product can be used.   Don’t use a systemic product when the plant is flowering – it can be harmful to bees.

Be sure the product you use is labeled for leaf miners and follow the instructions carefully.

Horticultural oils or neem oil can be sprayed every ten days. This smothers the eggs and interrupts the insect’s life cycle. Don’t use these oils when temperatures are above 80°F as this can burn the plants.  Spray both upper and lower leaf surfaces to the point of runoff.

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About the author: Alaine has been working way too hard in horticulture since 1992, beautifying golf courses, resorts, and hotels. She is a part time landscape designer who works full time caring for a 28,000 square foot public garden. At home, she maintains her own 400 square feet plot. Alaine lives in northern Illinois - zone 5b.

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