Leaf miners are a major concern in greenhouse production because they have developed resistance to many commercially available insecticides and very few effective insecticides are currently available. The “overuse” of abamectin (Avid) has resulted in several leaf miner species that are now totally resistant to this insecticide/miticide. Common leaf miners that attack greenhouse crops are in the family Agromyzidae (Order: Diptera), including the chrysanthemum leaf miner, Phytomyza (= Chromatomyia) syngenesiae; columbine leaf miner, Phytomyza minuscula; pea leaf miner, Liriomyza huidobrensis; and serpentine leaf miner, Liriomyza trifolii. Leaf miners cause plant damage primarily when in the larval stage. They may feed only on specific crops or on a wide range of greenhouse-grown crops, depending on the species.
Leaf miner larvae feed between the leaf surfaces in the mesophyll layer of cells, creating either blotches or winding, serpentine mines or trails. Based on the species, leaf miners may feed in different sections of the mesophyll layer. Furthermore, the mine pattern and location may vary, depending on the leaf miner species, stage of leaf development, and host plant.
Damage is mainly aesthetic or visual, rarely killing plants. However, a heavy infestation may impact plant salability. Under certain conditions, leaf miner larvae may tunnel into leaf stalks or into plant stems. An adult female may also cause damage by puncturing leaves as she inserts her ovipositor (egg-laying device). This creates white specks on the surface of leaves. Females may puncture both the upper and lower surface of leaves, depending on the species. Leaf puncturing can reduce photosynthesis and kill young plants. Also, these punctures may provide entry sites for diseases such as bacterial leaf spot on chrysanthemum. Leaf miners are generally host specific, and this trait can help identify them.
Leaf miner adults are 2- to 3.5-mm-long, shiny, black flies with yellow markings on the abdomen. They resemble fruit flies. Adult females lay single translucent, white oval eggs into leaf punctures created by the ovipositor during probing. Both the female and male feed on the sap that exudes from these wounds. Each female can lay an average of 60 eggs in a 2- to 3-week lifespan. The number of eggs laid depends on the food source and temperature. Eggs hatch into bright yellow to white larvae, or maggots, that feed on the mesophyll layer of cells, creating mines within the leaf. The larvae may either mine the top of the leaf, the bottom, or both, depending on the species. Temperature, host plant, leaf position, and age can all influence larval development.
Mines enlarge in size as the larva grows or molts to the next instar. The mine pattern, location, and plants attacked depend on the species. There are three to four larval instars that take about 5 to 8 days to develop prior to pupation. The last larval instar cuts a semicircular slit in the leaf and typically drops to the soil to pupate. Pupae are oblong, and brown to gold. Leaf miners require darkness to pupate, so they are typically located deep in the soil. Adults emerge from the pupa stage in 9 to 10 days. A generation may be completed in 16 to 24 days, depending on temperature.
Insecticides may be used to manage leaf miners; however, because a number of species have developed resistance to several commonly used insecticides, this has complicated control. Also, larvae are well protected within the leaf tissues, thus escaping insecticide contact. Insecticides that are primarily used for controlling leaf miners are either insect-growth regulators such as cyromazine (Citation), which target the larval stage, or materials with translaminar activity, including abamectin (Avid), spinosad (Conserve), and acetamiprid (Tristar). Products with translaminar properties are effective against the larvae, as these materials are can enter the leaf and kill the larvae. Pyrethroid-based insecticides are useful against the adults, including permethrin (Astro), bifenthrin (Talstar), cyfluthrin (Decathlon), fenpropathrin (Tame), and esfenvalerate (Mavrik); however, these materials are generally not effective on the larvae. The leaf miner numbers and the occurrence of overlapping generations influence the frequency of insecticide applications needed. Spray in the morning, when females are laying eggs; this action may disrupt their behavior. The problem of insecticide resistance has led to leafminers emerging as a major pest--once again.