EDITOR’S NOTE: This column was originally published May 18, 2017.
As nature enthusiasts, most gardeners take pleasure in observing the varied colors, patterns, shapes, and textures of the flora and fauna that inhabit their yards. One such discovery may not be the work of art you would expect from Mother Nature, as you notice the light green serpentine designs on some of your garden leaves.
Like a mountain path, the narrow squiggles are characteristic trails left by a group of insects known as leaf miners, which includes species of moths, sawflies, flies, and beetles. With many different types of leaf miners, the diversity, appearance, and habits are extremely variable, yet the plant damage they cause is noticeably similar.
Caused by their larva feeding within the layers of the leaf tissue, the damage may look serious, but is rarely severe enough to kill the plant, unless there is repeated infestation that might stress and weaken it. However, significant “mining tracks” can reduce the plant’s ability to photosynthesize, degrade its edible parts, and increase its susceptibility to diseases. In addition, not all leaf miners squiggle their way through leaves. Similarly, colored light green transparent blotches may also be signs of other species of leaf miners munching their way to maturity.
Adults target a variety of vegetable and flower host plants to lay their eggs. Spinach, lettuces, cucumbers, squashes, tomatoes, beans, begonias, impatiens, and marigolds provide prime feeding surfaces, as larvae burrow and tunnel into the foliage, leaving a transparent track as they forage. They will also chew their way through tougher shrubs and trees, like boxwood, alder, and citrus. But, what is there to eat between the skins of a leaf? A juicy substance called mesophyll fills this inner space., which is full of chloroplasts, the green engines of photosynthesis.
If you look closely, you can often see the larvae, as a dark, 1/5-inch to ⅛-inch speck at the end of one of the trails. Additionally, these young insects are very flat, an adaptation that enables them to feed inside the leaf. To complete their metamorphosis, they leave the “mines,” falling to the ground to pupate. With a two-week life cycle, there may be three to five generations per year, depending on the weather and climate. Many species will overwinter as pupae in the soil, emerging in late spring to resume the cycle.
The question ... to control or not to control? Most leaf miner species have many predators and disease organisms that provide natural population controls. Therefore, applying insecticides is not always the best way to handle a leaf miner problem since conventional insecticides will not only kill the leaf miners, but also the beneficial predators. Thus, it is important to regularly check leaves for mining trails, so as to rid the damage manually by picking or cutting, especially if there is a history of attacks.
Additional tips to prevent serious damage include:
♦ Planting resistant species or varieties;
♦ Providing proper care to keep plants vigorous and healthy;
♦ Covering plants with floating row covers to prevent the adults from laying their eggs;
♦ Removing infested leaves, especially edible ones, as it rids existing leaf miners before they become adults and can lay more eggs; and as a last resort; and
♦ Spraying those plants that are susceptible in early spring with an insecticide, such as neem.
Despite being regarded as a pest by many gardeners and farmers, sometimes it takes changing your perspective to ease your gardening frustrations into an appreciation for the interesting aesthetics that may be residing in the smaller facets of your garden.
Once observed, remember to destroy the inhabited leaf.
U. (Ed.). (2016, September 20). How to Manage Pests. Retrieved February 23, 2017, from ipm.ucanr.edu/PMG/GARDEN/VEGES/PESTS/vegleafminers.html.
Wold-Burkness, S., & Hahn, J. (2017). Leafminers in home vegetable gardens. Retrieved February 23, 2017, from www.extension.umn.edu/garden/insects/find/leafminers-in-home-vegetable-gardens.