Leaf miners is the classification assigned to the larvae a variety of insects. This includes moths, flies, wasps and beetles. These insects are attracted to garden foliage, and the leaves of junipers, arborvitae and birches in North America. The larvae live inside of trees and plants within the leaves. Leaf miners consume the inner plant tissue located between the lower and upper sides of the leaves.
The Citrus Leaf Miner
Citrus leaf miner larvae create shallow mines or tunnels in new leaves on citrus trees including oranges, lemons, grapefruit, mandarins, limes, calamondin and kumquat. Despite originating in Asia, citrus leaf miners are common throughout the majority of southern California and are a troublesome pest.
The citrus leaf miner was first seen in Australia during the 1940s. During the 1970s, the insect traveled all over the world where citrus was grown in abundance. In 1993, the pest appeared in Florida, traveled west, then eventually invaded Mexico. The insects reached California during the 1990s.
Identifying Leaf Miners
There are a lot of different insects in this classification. The name was derived from the larval stage when the insect consumed and mined the inside of the host plant’s leaves. There is substantial variation regarding the habits, appearance and diversity of leaf miners. In most cases, the insect looks like a black fly. The best way to identify them is by the damage caused to the plants.
The adult leaf miner is tiny, can be hairy, and is approximately one-quarter of an inch in size. The larvae are extremely flat, enabling them to feed inside of the leaves. When the larvae are feeding, the green tissue in the leaves is consumed. This leaves a meandering and thin trail with a cover. When multiple leaf miner larvae are feeding on the same leaf, the tunnels often connect, with an appearance of big spots of blemishes.
A citrus leaf miner is a tiny moth with a light coloration, and a maximum of one-quarter inches in length. The forewings are a whitish-silver iridescence with white and brown markings. The tip of each wing has a distinctive black spot. The body and hind wings are white. During the larvae stage, the citrus leaf miner is only found in citrus leaves, and similar plants.
As the development of the larva continues, a trail of feces is left in a thin line below the surface of the leaves. This line is how the leaf miners are generally identified. Eventually, the larvae will make their way from the inside of the leaves and move toward the edges. The insects roll the leaf around their bodies to prepare for adulthood.
Leaf Miner Life Cycle
Adult leaf miners appear between late April and the middle of May. The pupae spend the winter months in either plant debris or the soil. Once the adults have mated, the female lays extremely small eggs in clusters or singly. The eggs are left on the underside of the leaves. When first hatched, the larvae do not have any legs. They immediately burrow into the leaves and start feeding.
Once mature, the leaf miners begin chewing through the surface of the leaves, drop to the surface of the soil, and start pupating. The complete life cycle is between 30 and 40 days, frequently resulting in two to three generations overlapping each year. The complete life cycle of the leaf miner includes the egg, grub or larvae, cocoon or pupa, and the adult. The pupation for some occurs in the leaves, but the majority pupate in the soil.
What Does Leaf Miner Damage Look Like?
Leaf miner damage can be identified by the following:
- Squiggly yellow lines in the leaves
- Pale blotches on the leaves
- Tunnels on the leaves
- Brown leaves
- Prematurely falling leaves
- Slow plant growth
- Distorted and rolled leaves
Due to the diversity of leaf miners, there is a wide variety of host plants such as cannabis and vegetables including lettuce, peas, cabbage, beans and tomatoes. Leaf miners also attack flowering plants including petunias, begonias, impatiens, dahlias and marigolds, and shrubs or trees. This includes cottonwoods, aspens, boxwood, evergreens, elms, azaleas and birch.
Leaf miner damage is different for the larval and adult stages. The adults cause different types of damage such as tunnels and pale blotches on the leaves resulting from feeding. If the infestation is bad, the leaves can turn brown, then prematurely fall prior to the end of summer. The insects only cause cosmetic damage, with no serious injuries to the plants. This pest is unable to harm or bite humans.
During the larval stage, there is rarely any impact to the growth of the plant. The only time the plant can be killed is during the seedling stage. If enough leaf miners are present, plant growth is substantially slowed. The only way citrus leaf miners can survive in the larvae stage is in the shiny, young and tender leaves of citrus plants and similar species.
Unless the population is extremely high, the older, hardened leaves are not attacked. The mining takes place within the interior of new leaves, resulting in a curled and distorted appearance. Mature trees with a minimum age of four can tolerate the damage caused to the new leaves due to the thick canopy of foliage. During this portion of the growing season, the effect on the fruit yield and tree growth is negligible.
The population is greater in extremely young trees due to the lack of mature foliage. The growth of younger trees is often decreased. Even if there is a heavy infestation present in a younger tree, the chances of the tree dying is slim. The populations are suppressed in certain parts of California due to the heat in the summer.
Since the coastal areas are cooler, the population stays high throughout the summer and fall months. When leaf miners attack citrus trees, the appearance becomes unsightly. In many instances, the best option is leaving the trees alone since the natural enemies of these insects will consume the larvae. Attempting to control the infestation using insecticides is not recommended.
The infestation of leaf miners throughout numerous parts of the globe was established a long time ago. During the first two years, the amount of damage was high. The severity then decreased because natural enemies came to consume these pests. These enemies are still present, with their survival dependent on numerous types of leaf mining insects. Eventually, the increased population of enemies resulted in a declining population of leaf miners.
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How to Get Rid of or Kill Leaf Miners Naturally
There are several methods for killing leaf miners. Learning how to get rid of leaf miners naturally is the best defense including:
- Use your fingers to crush the tunnels to eliminate larvae
- Beneficial bugs
- Prune the plants
- Use sticky traps
All plants and trees should be monitored regularly. The larvae can be killed by crushing the tunnels using your fingers. Early detection is incredibly important to prevent the population from becoming too large. When infested leaves are seen, they should be removed, crushed, and discarded away from the plants. Beneficial bugs are also a good option for killing the leaf miners.
Diglyphus isaea wasps can be purchased from a reputable nursery. As natural enemies of the insects, most of the population will be consumed quickly. Understanding how to get rid of leaf miners is often dependent on learning more about the predators. Pruning the plants is a good way to determine if there is an issue. Eliminating leaf miners is the only way to prevent an infestation.
Plants should be examined every day. Look for any signs that leaf miners have been burrowing throughout the leaves. If the leaves have already been attacked, prune the plant by removing the leaves, and throwing them away. If the damage to the leaves is minor, squeezing the leaves gently will kill the insects without requiring removal of the leaves.
A sticky trap is a card covered using adhesive. The trap can then be placed near the plants on a stick or hung. Many of traps have bright colors since pests are attracted to blue and yellow. The adhesive causes the adult to become stuck. This makes it impossible for them to lay eggs. Once full, the traps are simply discarded. Sticky traps can be purchased online, at local garden centers, or home made.
How to Prevent Leaf Miners Naturally
- Regular examination of plants
- Fertilize and compost regularly
- Plant trap crops
- Cover with row covers to eliminate access
- Remove the infected leaves
Examine plants on a set and regular schedule to identify the issue quickly. Plants can be protected by covering them with row covers. This will stop adult leaf miners from laying eggs by denying them access to the leaves. All leaves infested with larvae need to be removed and discarded away from the plants. Plants should be watered regularly to keep them vigorous and healthy.
Fertilizing and composting are important for the maintenance of plant health. The process is slightly different because each plant has individual needs including cannabis, herbs, vegetables and fruits.
One of the best ways to prevent an infestation is by using floating row covers. These covers can be used with another method to eliminate leaf miners from plants and gardens. Once reaching adulthood, the insects are capable of flight. The covers make it impossible for them to get to the plants. This means they are unable to deposit any eggs. Mature larvae will be unable to drop to the ground and begin burrowing. This effectively disrupts the life cycle. Remove any affected leaves by hand before putting down row covers.
The row cover will then act as protection to stop the leaf miners from returning. Local hardware stores sell PVP pipes making ideal support for the framework of row covers. This is the best way to protect your plants. Row covers also offer plants protection from light frosts, but do not offer adequate protection for a heavy frost.
Another excellent option to keep leaf miners away from plants is trap crops. A trap crop is a plant the insects find enticing. The crops need to be planted as close as possible to offer enough enticement to keep the pests away from the other plants. The best trap crops to prevent leaf miners naturally include lambs quarter, columbine and velvetleaf. Planting more than one type is recommended.
Leaf Miner Management in Greenhouses Using Biocontrols
Dacnusa is a parasitic wasp. Eggs are deposited by this predator into the larvae of young leaf miners. The lifecycle of the wasp is completed in the host body which kills it. New wasps emerge from the leaf miner in approximately 17 to 19 days when the temperature is 68 degrees Fahrenheit. You need to release between one and three adults biweekly for each square yard.
The rate for a curative release is between two and five adults biweekly for each square yard with two to three total releases. Tips for dacnusa release include:
- Introduce dacnusa when leaf miners are initially detected
- Leaves need to be monitored for parasitism every two to three weeks
- The predators must be equally placed in your greenhouse
- Leave the tube in your greenhouse until empty
- You can temporarily store adults vertically at a temperature of 43 to 50 degrees Fahrenheit and a humidity level between 50 and 90 percent.
- Put samples of leaf miner pupae into a glass bottle or jar to see if a wasp or leaf miner emerges. You can count the numbers of leaf miners and wasps emerging to determine the parasitism percentage.
The diglyphus isaea parasitic wasp will sting the larvae of small leaf miners for parasitization. Oval eggs are then laid next to the larvae. The predator larvae will consume the leaf miner larvae after emerging. If the density of leaf miners is lower, the efficiency of locating prey improves. For preventative measures, introduce one or two wasps triweekly for each square yard.
For a curative release, introduce two to four adults biweekly for each square yard two or three times. Tips for diglyphus use include:
- Carry the open tube through the vegetation for an equal distribution
- When the tube is nearly empty, leave it in your greenhouse
- The temperature needs to be between 75- and 90-degrees Fahrenheit
- The humidity level should be approximately 80 percent
- Check for parasitism every two or three weeks just like with dacnusa
- If the population of leaf miners is low, use a combination of dacnusa and diglyphus
Sanitation is extremely important for controlling this pest. All plant debris must be destroyed, and all infested leaves removed. Leaf miners may use broadleaf weeds as hosts. Good weed control must be practiced both outside and inside of your greenhouse. Adult leaf miners can also be trapped using yellow sticky traps.