Leaf miner

A leaf miner is a larval stage of various insect species that live and feed within the tissues of a plant's leaves. The term does not describe a single taxonomic group, but rather a feeding behavior known as "leaf mining" that has evolved independently across several insect orders. Leaf miners are considered both ecologically significant and economically important because of their role in ecosystems and their impact on agriculture and horticulture. Leaf miners consume the inner tissues of leaves while leaving the outer epidermal layers largely intact. This results in distinctive patterns or "mines" on the leaves, which can appear as winding trails, blotches, or tunnels. Leaf mining is an ancient ecological strategy that has been employed by insect larvae since at least the beginning of the Permian period, around 295 million years ago.

Leaf mining behavior is observed in multiple insect groups, including:

Each group contains numerous species with larvae that are specialized for leaf-mining on their preferred host plants.

Adult leaf mining insects typically lay eggs on or within the surface of a host plants leaf. When the larvae hatch, they burrow into the leaf and begin feeding between the epidermal layers. Much like woodboring beetles, leaf miners are protected from many predators while feeding within the tissues of the leaves, selectively eating only the layers that have the least amount of cellulose. After completing larval development, the insect pupates either inside the mine, on the leaf surface of the leaf or within the soil below the host plant depending on the species. Emerging adult insects can then continue their cycle.

Plants have developed a variety of defense strategies to reduce damage from leaf miners. These defenses can be structural, chemical, or physiological and may act either directly against the larvae or indirectly by attracting natural enemies.

The pattern of the feeding tunnel and the layer of the leaf being mined is often diagnostic of the insect responsible, sometimes even to species level. The mine often contains frass, or droppings, and the pattern of frass deposition, mine shape, and host plant identity are useful to determine the species and instar of the leaf miner. Some mining insects feed in other parts of a plant, such as the surface of a fruit or the petal of a flower.

Leaf miners are regarded as pests by many farmers and gardeners as they can cause damage to agricultural crops and garden plants, and can be difficult to control with insecticide sprays as they are protected inside the plant's leaves. Spraying the infected plants with spinosad, an organic insecticide, can control some leaf miners. Spinosad does not kill on contact and must be ingested by the leaf miner. Two or three applications may be required in a season. However, this will have harmful ecological effects, especially if sprayed when bees or other beneficial arthropods are present.

Leaf miner infection of crops can be reduced or prevented by planting trap crops near the plants to be protected. For example, lambsquarter and columbine will distract leaf miners, drawing them to those plants and therefore reducing the incidence of attack on nearby crops. This is a method of companion planting.