How to Identify and Control Leaf Miner
The leaf miner chews trails through leaves, disfiguring foliage and leaving it open to fungal disease. Horticultural expert Lorin Nielsen discusses how to identify and control these damaging pests in your garden.
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You might have noticed the winding white trails of the leaf miner on some of your favorite plants. Maybe they suffered as a result of the loss of leaf tissue, or maybe they recovered. But letting leaf miners have their way in your garden comes with risks.
What makes these trails in leaves is the leaf miner larvae. They not only remove plant matter as they feed but have the potential to spread diseases in the process. Plants can make it out unscathed, but it’s best to stop them when you can.
This piece is dedicated to explaining the leaf miner, and examining how to handle them in your garden. We’ll identify the most common types of leaf miners, and talk about integrated control methods that keep your plants happy.
What Are Leaf Miners?
Thousands of different fly or moth larvae are considered to be leaf miners. Each variety has its own favored plant type.
Most of these are from the Lepidoptera (moth), Symphyta (sawfly), or Diptera (fly) families. However, there are a few exceptions from other genera. Their life cycles are all similar, but there are subtle differences.
Types of Leaf Miners
While we won’t name every single one of the thousands of leaf miners, we’ll cover the basic categories of many. This includes the ones you’re most likely to find in your garden.
Generally, the preferred host of the leafminer will determine which you’re dealing with. Thankfully, leafminer treatment is similar across families and species.
- Pegomya hyoscyami (spinach / beet leafminer): The pale yellow larva of this species has a dark black stripe down its back. Host plants include chickweed, plantain, nightshade, lamb’s quarters, Swiss chard, beets, and spinach.
- Douglasiidae (Douglas leafminer): The pale yellow to cream larvae in this family prefer borage, and plants in the mint and rose families.
- Gracillariidae (citrus leaf miner, Horse-chestnut leaf miner): The larvae in this family prefer citrus trees, horse chestnut, and a wide range of other dicots.
- Gelechiidae (tomato leaf miner): Members of this leafminer family have a range of anatomies, but many have a cream-colored body with dark brown or black vertical stripes. They prefer tomato, eggplant, potato, and other nightshades.
- Liriomyza sativae (vegetable leaf miner): The leafminers in this species are stout, and cream to lime green. Their preferred hosts include over 20 common garden vegetables.
- Liriomyza trifolii (American serpentine leaf miner): The larvae of this species are similar to those of the vegetable leafminer. They prefer many plants, including chrysanthemum, gerbera, celery, sweet pepper, and potato.
- Nepticulidae (pygmy leafminer): These leaf miners are markedly ridged, and range from white to yellow, to orange. Host plants include trees, some mine seeds and bark.
- Tenthredinidae (sawfly leaf miner): Anatomically diverse larvae in this family may have stripes or spots, and range from yellowish green, to brown. Host plants include hundreds of plants as this family encompasses several subfamilies.
- Tischerioidea (trumpet leaf miner): Another family with intensely ridged leaf miner larvae that are yellow or green. Preferred hosts of these 500+ species range widely.
Life Cycle
There are slight differences, but the basic life cycle is the same for all leaf miner species.
Female adult leaf miners lay eggs on leaves in early spring, or inside leaf surfaces. Sometimes this creates a small raised spot on the leaf’s surface. One female may lay up to 250 eggs at once.
The egg stage lasts ten days or less. In warm weather, the leafminer eggs hatch into larvae. When the larvae hatch, they tunnel through the leaf tissue. As the larvae feed, they leave squiggly trails or lines on leaves that leaf miners are associated with.
This feeding phase lasts for two to three weeks, depending on the species. Once the larvae near pupation, they chew through the skin of the leaf and drop onto the ground below. Then they burrow an inch or two beneath its surface.
If the weather is too cold for them to survive, the larvae enter dormancy and overwinter in the soil beneath the plant. Otherwise, they form a pupa and begin their final transition out of the larval stages into adulthood.
It takes approximately 15 days to metamorphose from pupa to adult. When the leafminer larvae finish pupating, the adult moth or fly will dig its way back out of the soil and begin the cycle again.
Symptoms
Throughout the United States, leaf miners are common, although they aren’t as financially destructive in the northern states. Warmer climate regions house commercial farms which suffer major issues. A wide range of plants are subject to leafminer damage. Grasses aren’t likely hosts, but leaf miners pupate beneath grassy soil.
The juicy inner tissues of leaves are subject to leaf miner damage, but they prefer leaf matter that has less cellulose. They may chew through a leaf’s vein to get to more leaf on the other side, but they tend to ignore the veins in favor of sweeter, softer plant tissues. As the bug’s tracks are created, the affected area becomes a trail of dead tissue.
Particular species of leaf miners are incredibly selective. Citrus leafminers are an excellent example. While the citrus leafminer may not cause extensive damage to a tree, they leave open pathways for fungal or bacterial growth on the leaves. For farmers, the damage caused by citrus leafminer makes portions or the entire crop unsellable.
Additionally, spinach leaf miner damage makes the leaves unpalatable and potentially unsafe to consume. Appearances of leaf miners are a sign it’s time to act.
Leafminer species don’t limit their feeding to edible plants. Oak and aspen leaves are subject to leafminer problems, and leaf miner boxwood issues are common in hedging. A number of flowering plants, trees, and shrubs are susceptible to damage as well by the vegetable leaf miner, citrus leafminer, and American serpentine leaf miner.
While in most cases this leaf miner damage won’t kill a plant unless it’s heavily infested, they affect plant growth and cause unsightly views for gardeners.
Leaf Miners As Disease Vectors
Learning how to prevent leaf miners and understanding leaf miner control is also important because infested plants can be subject to diseases.
As the leaf miner larva leaves the plant, the leaf miner trails are vacated, making way for fungal and bacterial disease to colonize them. Many of the diseases associated with leaf miner larva are of the soft rot variety – root rot is one of them.
These are highly communicable, and infected plants may have to be destroyed to control the spread.
Control
The best mode of control for infestations will be an integrated pest management strategy that includes cultural, biological, and insecticidal tools. Let’s talk about those so you can keep that citrus leafminer, serpentine leaf miner, or some other kind off your citrus trees or other plants.
Cultural Control
The first step in environmental control is actually the simplest. If you see a leaf that appears to house one of these tiny pests with their winding leaf mines, follow where they lead. Miners within bug tracks can be squeezed with your thumb and forefinger. You may be able to kill the larvae inside the leaf this way.
Healthy plants are the least damaged by these little chewing pests, so ensure your plants are well taken care of. Regularly fertilize your plants, keep them pruned well, and provide compost or other good soil for them to grow in. Plant health is paramount!
Related to the health of your plants is keeping the garden free of plant debris to throw a kink in the entire life cycle of these miners. Many overwinter in crop debris, along with a few different diseases.
Biological Removal of Leaf Miners
Beneficial insects come into play here as well. Certain species of nematodes kill pupae in the soil, preventing the emergence of egg-laying adults, like flies or tiny moths that keep the life cycle going. Adding beneficial nematodes to your soil will help eliminate those, too. Treat your soil in temperate seasons at least twice to establish a productive colony of nematodes.
A particular species of parasitoid wasps, Diglyphus isaea, finds the larvae while they’re in their trails. These tiny wasps lay their eggs inside the leaf miner larvae. As they hatch, the larvae are consumed from within. Learn how to grow lettuce and plant chrysanthemums to bring these helpers into your garden. They’re harmless to humans too!
Insecticidal Removal of Leaf Miners
It’s hard to kill leaf miners by spraying leaves with insecticides because those in the larval stage are protected inside infected leaves. Because of this, spray solutions have limited effect.
However, a thorough coating of all plant surfaces with neem oil does have some effect. Not only does neem actively fertilize the plant, but the naturally occurring azdirachtin compounds in the oil slowly kill leaf miners, especially with repeated spraying.
Application of Bacillus thuringiensis sprays, also known as BT, helps with some larval issues. These bacteria poison leaf miner larvae if they come into contact with them.
Prevention
The most important part of leaf miner treatment is not knowing how to get rid of leafminers, but prevention. There are a few different options for preventing the assortment of flies and moths that produce leaf miners from causing future harm.
Use a floating row cover to prevent adult flies from reaching your plants. If you’re concerned about citrus leaf miners or leaf miner on citrus trees, cover the tree with the same fabric used for row covers and affix it at the trunk with some string. If they can’t reach the plant, they can’t lay their eggs. And if they can’t lay their eggs, the leaf miners can’t leave trails!
A yellow sticky leaf miner trap is also a solution. Hang these near or on plants where leaf miner damage is likely to happen. Flies and moths will get stuck to them and will die off. Note these can also trap natural enemies of leaf miners. Be strategic when you place them.
Certain species of leafminer respond well to pheromone traps. Citrus leafminer control can be achieved with the use of these lures, for instance. The pheromones lure adults to the trap, and they get stuck within and die. These traps don’t have the same risk of luring in natural enemies like sticky traps do.
Frequently Asked Questions
Does insecticidal soap work on leaf miners?
No, this pesticide hasn’t proven to be an effective treatment.
Can you eat leaves with leaf miners?
You can, but it’s not recommended as they can spread disease while they feed. This is especially true with spinach.
Will diatomaceous earth kill leaf miners?
Powders like diatomaceous earth are inefficient at eradicating leaf miner larvae because the larvae just don’t come into contact with it. It can be spread dry over the soil surface underneath a plant and dusted onto the plant’s leaf surfaces. Emerging larvae that are preparing to drop into the soil to pupate may come into contact with it that way, but it is not as effective as other methods.