“Leaf miner” is a general term for insects that literally mine fleshy leaves for food. They do so in the form of their minute larvae, which are small enough to create tunnels within the nutrient-rich tissues of leaves. This group of pests is largely composed of lepidopteran (moth), dipteran (fly), hymenopteran (sawfly), and coleopteran (beetle) larvae. The tunnels they create, along with their droppings, are distinct enough to identify them at the species level.
Many leaf miners are able to feed on plants with chemical defenses by avoiding the tissues that contain toxic compounds. By selecting which leaf layers to feed on, they are able to graze through a seemingly large section, creating a maze-like pattern of damage in the process. This can ruin the visual appeal of the leaves, thereby lowering the plant’s market value. Agricultural crops that are heavily damaged by leaf miners can lead to major financial losses.
Insecticides are seldom effective against leaf miners as they work upon contact. When the larvae are safely tucked in their tunnels within the leaf blade, they are protected from external chemicals. Planting herbs and shrubs that repel leaf miners or attract their predators are thus recommended to decrease the chances of infestation. Consider using the species below as companion plants for vulnerable crops.
1) Rosemary (Salvia rosmarinus)
An evergreen shrub with tough features, rosemary is cultivated for its needle-shaped and leathery leaves. These are thick enough to withstand cool and harsh conditions in zones where temperatures may drop to -20˚C (-4˚F). Given its preference for dry substrates, this herb can withstand periodic droughts. It is an ideal ornamental addition to xeriscaping projects in the home garden.
A rosemary hedge can be grown around leafy herbs that are often targeted by leaf miners. It should serve as a physical barrier that deters pests and discourages them from approaching any adjacent plants. The rosemary leaves are too tough and full of potent phytochemicals, making them highly undesirable and troublesome to leaf-eating insects.
In the absence of actual rosemary shrubs, this herb’s essential oil can be used as an organic treatment to prevent pest infestations. If you manage to obtain a horticultural extract of its oil, you may add it to a pesticidal spray. This should help deter many of the adult forms of leaf miners, preventing them from laying their eggs in susceptible leaves. Unfortunately, the oil is not able to permeate through the leaf tissues and cannot eliminate larvae within their tunnels.
2) Marigold (Tagetes spp.)
Marigolds are often cultivated as ornamental plants for the eye-catching colors of their blooms. These culturally important annuals and perennials are rapid growers with a knack for spreading quickly in almost all types of soil. The most popularly grown species are French marigold (T. patula), African marigold (T. erecta), and the signet marigold (T. minuta).
Marigolds are fantastic companion plants for vegetables and fruit-producing crops. Apart from attracting various pollinators and beneficial insects, they harbor essential oils and compounds that deter harmful nematodes and destructive grazers. Healthy specimens are generally able to keep leaf miners away due to their chemical makeup and the predatory insects they attract.
Keep in mind, however, that weak marigolds are just as attractive to leaf-mining insects as valuable crops. The trick lies in ensuring your marigolds are in great shape. The pest-repellent effects of marigolds also extend to their companion plants, preventing pest infestations that would compromise their health and compound their susceptibility to leaf miners.
3) Lavender (Lavandula spp.)
Extensively grown as ornamental plants and as sources of economically important essential oils, many lavender species have had global renown for centuries. The English lavender (L. angustifolia) is the most heavily cultivated Lavandula and is the sole crop of many lavender fields throughout Europe. Drought-tolerant, sun-loving, and pest-resistant when grown in optimal conditions, it yields a treasure trove of garden benefits.
Velvety, narrow, and full of potent phytochemicals, lavender leaves are rarely of interest to leaf miners and other leaf-eating pests. Their mere texture and small size can minimize the efficiency with which leaf miners can normally tunnel through and extract nutrients from leaves. Moreover, as the winged adults of many leaf miners tend to avoid laying their eggs on lavender leaves, Lavandula spp. are seldom plagued by infestations.
Stands of lavender plants can be grown around plots with susceptible herbs. Troublesome leaf miners, which gradually infest plants that are adjacent to those with their existing infestations, may be deterred by a barrier of unfavorable leaves. Planting lavender species can also increase the densities of parasitoid wasps, which will readily attack the egg and larval forms of leaf miners.
4) Dill (Anethum graveolens)
Widely cultivated for its flavorful and aromatic properties, dill is a slender herb with threadlike leaves. Leaf miners are mostly disinterested in these leaves for many reasons. For one, they are too short and narrow to accommodate their winding tunnels. Unlike the leaf blades of many vegetable-producing plants, dill leaves do not have a large surface area for colonization. Moreover, they are attached to hollow stalks – devoid of the juicy tissues favored by leaf miners.
If you’d like to attract more predatory insects that can feed on existing leaf miner colonies, planting a few stands of dill should also do the trick. Like lavender, this herb is attractive to parasitoid wasps. If you grow it next to vegetable plots, the visiting wasps should quickly take to your vegetables in search of insect eggs and larvae to feed on.
Apart from being a natural form of defense against leaf miners, dill is useful as a natural antibiotic and as a source of essential oils. Its oil has shown promise as an insecticide against lepidopterans, some of which produce leaf-mining larvae.
5) Fennel (Foeniculum vulgare)
Like dill, fennel is known for attracting parasitoid wasps. Drawing these into your garden is truly the best natural means of controlling leaf miner infestations. These wasps, which are considered the primary enemies of leaf miners, are able to track and find them by following their mines. They also feed on the eggs of these pests, preventing them from hatching on and damaging foliage.
Fennel produces blooms, nectar, and pollen with adaptations for specifically targeting beneficial insects. It should be planted close to crops that are known for being highly attractive to leaf miners. A healthy stand of fennel should help protect most vegetables, though it may inhibit the production of blooms, seeds, and fruits. Unfortunately, despite being so great at repelling pests, it simply isn’t the best companion plant.
To make the most of the benefits of F. vulgare without compromising the growth of your other plants, it would be best to cultivate it in dedicated plots or in separate pots. Its blooms should still attract many parasitoid pests to your garden. These are likely to cover a short distance to protect any neighboring vegetables and herbs.
6) Garlic (Allium sativum)
One of the world’s most economically important root crops, garlic is highly potent, aromatic, and flavorful due to a complex mixture of phytochemicals. Rich in sulfur-based compounds, its oils are known for having pest-repellent properties. The strong scent of the bulbs, which is released when its cloves are mechanically damaged, repel both small pests and large grazers.
As garlic extracts are able to deter both flying and crawling insects, it can be added to a spray for the treatment of susceptible plants. The juices should stave off any larvae and adult insects that are found on the surfaces of the leaves, but leaf miners that have bored into the underlying tissues may remain unharmed. Its oils must be used in conjunction with other broad-spectrum treatments to be fully effective.
Note, also, that the leaves of garlic may themselves be infested with leaf miners. It would thus be prudent to grow this natural insecticide alongside other herbs that are seldom targeted by leaf mining insects. As garlic should aid in keeping pests away from its companion plants, these should likewise aid in keeping leaf miners away from its leaves.
7) Common yarrow (Achillea millefolium)
A flowering perennial that produces impressive inflorescences, the common yarrow is a fantastic ornamental herb. With a scent that is similar to that of chrysanthemums, it attracts a diversity of insects. Many of these are highly beneficial in both flower and vegetable gardens.
Also known as devil’s nettle, old man’s pepper, and soldier’s woundwort, the common yarrow is distinguished by its featherlike leaves. Composed of multiple leaflets, those close to the base of the stem tend to be the largest. Blooms are produced from spring to late summer or early fall. Some birds use yarrow sprigs to reinforce their nests and improve their resistance to parasitic infestations.
Parasitoid wasps, which readily feed on leaf miners, are drawn to the features of common yarrow. This herb is thus an ideal choice for gardens that are often plagued by these troublesome pests. Note, however, that yarrow naturally attracts many other leaf-eating insects. The plant’s main mode of controlling infestations and encouraging pollination is by appealing to both these troublesome pests and their predators.
8) Variegated plants
Researchers have begun to look at the evolutionary benefits of variegation and its possible implications relative to pest colonization. If you look closely at leaves that have been damaged by leaf miners and undamaged leaves with natural variegation, you may find some similarities. Botanists specializing in tropical plants have proposed that variegation may actually be a form of mimicry. By developing unique patterns of color, the leaves feign colonization by a particular pest.
For example, variegated Caladium steudnerifolium, a tropical plant in the understory of Ecuador’s rainforests, may develop a variegation pattern that looks similar to the mazes left by mining moths and caterpillars. This pattern appears to be effective at repelling these pests, which perceive the leaves as previously colonized or ill. Non-variegated leaves of the same species, on the other hand, may be significantly more damaged as they continue to be targeted by leaf miners.
Although the development of variegated leaves seems counterproductive, leading to a reduction of chlorophyll in the plant, it may actually be advantageous in the long run. Reduced efficiency is offset by the lower risk of pest infestation, increasing the plant’s chances of survival.
If you’re worried about leaf miners in your garden, you may want to test this theory of mimicry via variegation. By cultivating multi-colored varieties of ornamental plants, you might just fool most pests into thinking there’s no more room for them!
Using Decoy Patches or Trap Crops
Instead of growing a limited number of leaf miner-repellent plants, some vegetable gardeners opt to maintain patches of trap crops instead. These crops, which are highly attractive to leaf mining insects, draw them away from the more valuable plants (i.e. the “cash crops”). Essentially, these are plants that should be much more appealing to all the growth stages of leaf miners.
When selecting trap crops, keep in mind that their features will depend on those of the plants you wish to protect. Generally, however, broadleaf plants are the best candidates because the adult forms of leaf miners are drawn to leaves with a large surface area. These insects lay their eggs only on leaves that can sustain the metabolic needs of their young.
Vegetable growers agree that some of the best trap crops include columbine, velvetleaf, lamb’s quarter, nightshade, pigweed, citrus trees, henbane, and blackberries. If these trap plants are placed close to your valuable crops, the inviting foliage should distract them. A major downside, of course, would be that these plants can continue to sustain the life cycle of leaf miners in your garden.
Just about the same song is sung by every gardener who inevitably has to deal with pests. There are always upsides and drawbacks to every form of crop protection. At the end of the day, however, the most ecologically-friendly and sustainable ones will always be the best long-term and guilt-free solutions!
