By Katie Duttweiler

Plant Pathologist

Iowa State University

Last week I noticed tan, winding steaks on the leaves of my marigolds. Initially I was excited at the thought of discovering a new marigold with variegated leaves But, upon closer investigation, I realized the unusual squiggles on my marigold leaves were actually the result of a common insect pest, leaf miners.

Leaf miner is a broad term for insects that, as their name implies, mine and consume the innards of leaves. Tan-colored squiggles in foliage develop as leaf miners eat the green leaf tissue while leaving the upper and lower epidermis of leaves intact. Different species of flies, moths, beetles and sawflies can be leaf miners, but in all cases it is the larvae (including fly maggots, moth caterpillars, beetle and sawfly grubs) that play the role of miner. Some leaf miners have indiscriminate tastes and will eat practically anything that is green and photosynthesizes. Other leaf miners have more particular palates and will only indulge on one specific plant species. 

Leaf miners can be found on vegetables, herbaceous ornamentals, woody ornamentals and deciduous trees.  Almost all of our favorite herbaceous ornamentals are hosts such as ageratum, aster, calendula, chrysanthemum, dahlia, daisy, gerbera, gypsophila, marigold, petunia, snapdragon, sunflower and zinnia. 

Our vegetable garden is not better off as the list of hosts includes beans, carrot, celery, cole crops (such as cabbage and broccoli), cucumber, eggplant, leek, lettuce, melons, peas, pepper, potato, okra, onion and tomato. Leafy vegetables, for example beets, spinach and Swiss chard, are particularly at risk since the edible part of the crop is directly damaged. Even our woody landscaping is at risk with abrovitae, apple, aspen, azalea, birch, basswood, cottonwood, dogwood, elm, holly, oak and poplar listed as some of the hosts.

The leaf miner life cycle begins as the adult females lay eggs directly into the leaf.  From the moment they hatch from the egg, larvae meander through leaves while dining on the plant tissue. The tunnels get larger as the larvae mature and increase in size. Once ready to pupate, the larvae chew a hole through the leaf epidermal tissue and drop to the ground. Adults eventually emerge from the pupa and the cycle repeats. Depending on the environment, the life cycle generally takes 18 to 21 days, and during the summer there can be multiple generations. When in a warm environment, such as a greenhouse, the life cycle can repeat throughout the year.

The damage of leaf miners is not limited to serpentine tunnels in the leaves. Infested leaves can dry out, turn brown, and/or drop prematurely. Many adult leaf miners feed on leaf sap, which is accessed by puncturing the leaves. These feeding punctures can cause leaves to stipple. Leaf miners consume plant parts responsible for photosynthesis, which can thereby decrease crop production in severe infestations.

Severe infestations of woody ornamentals can lead to re-leafing. Some leaf miners cause blotching and blistering of leaves rather than tunneling. The wounds caused by egg laying, adult feeding and/or larval emergence can make the plants more susceptible to disease by providing an entryway for plant pathogens. Some leaf miners also can transmit plant diseases. For example, the soybean leaf miner can transmit bean pod mottle virus.

Fortunately, severe leaf miner infestations are generally limited to greenhouses rather than the home garden. In most cases, the leaf miner populations are too low to cause any damage to plant functions. Therefore, leaf miner damage is generally considered to be an aesthetic issue. 

But, since we garden for aesthetic reasons, it is important to be aware of the presence of leaf miners. When at a gardening center, carefully observe the foliage of plants you are interested in purchasing. Leaf miner infestations often begin at the nursery rather than your garden. When admiring your flowers and vegetables at home, take a closer look at the leaves.  If you observe the distinctive tan squiggles, remove and destroy infested leaves. Removal of weeds that are alternative hosts also helps decrease leaf miner populations. Insecticides are tricky to apply effectively and are not recommended for homeowners. Thankfully, Mother Nature, who gives us cold winters and natural enemies, often provides the most effective control of leaf miners.

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