A few leaf-mining flies are common pests of tomato plants, including Liriomyza sativae, L. trifolii and L. huidobrensis. These small yellow-and-black flies lay their eggs inside the leaves of tomatoes, where the larvae hatch and proceed to consume the nutrient-rich, chlorophyll-filled cells. Eventually, they meander to the edge of the leaf, where they drop to the ground and pupate. These pupating larvae are easy targets for predators and other biological controls, but foliage-applied pesticides can also destroy leaf miners.
Vegetable leafminer can cause significant damage to host plants. Damage is caused by larvae feeding under the surface of leaves and petioles. Typically, this feeding causes long, narrow 'mines' which appear as white or grey lines on leaves (and can also look like coils) widening towards the end. Multiple mines on an individual leaf greatly reduces the result in the photosynthetic ability of the plant. Damage caused by vegetable leafminers considerably reduces the growth and development of seedlings and young plants, and can lead to plant death.
The presence of unsightly leaf damage in ornamental plants can lead to reduced market value.
The most important aspect of leafminer management is conserving their natural enemies, which are often killed by broad-spectrum insecticides applied for other tomato pests. Reduce the risk of leafminer outbreaks by applying insecticides for fruit pests only when monitoring shows treatment is needed and by choosing insecticides that will not destroy the leafminer parasites.
Biological Control
Several species of parasitic wasps, particularly Chrysocharis parksi and Diglyphus begini, attack leafminer larvae; left undisturbed, parasites often keep leafminers under control.
Cultural Control
Check transplants before planting and destroy any that are infested; leafminers reach damaging levels earlier when infestations begin on seedlings. Tomato varieties with curled leaves, such as VF145s, are less susceptible to leafminer damage and may provide suitable alternatives where leafminer damage is expected, as in fields adjacent to other infested crops. Where a series of tomato crops is planted in the same area, you can reduce early infestations in a new crop by removing old plantings immediately after the last harvest.
Organically Acceptable Methods
Biological and cultural controls as well as sprays of the Entrust formulation of spinosad are acceptable for use on an organically certified crop.
Monitoring and Treatment Decisions
A monitoring technique for leafminers in fresh market tomatoes is to place plastic trays about 12 by 15 inches in size beneath plants at several randomly chosen places in the field. Mature larvae that drop from foliage accumulate on the trays and pupate there, providing a measure of leafminer activity. A treatment threshold used experimentally for L. sativae and L. trifolii in southern coastal fresh market tomato fields is to treat when an average of 10 pupae per tray per day accumulates over a 3- or 4-day period. In all areas, do not treat unless pupae are present. Absence of pupae, even if new mines are present, indicates that natural controls are keeping leafminers controlled.
The dominant species of Liriomyza leafminers is in flux. All species, however, have high resistance to organophosphates, carbamates, and pyrethroids. If these types of insecticides are used, leafminer populations will increase. Treatment recommendations currently involve the rotation of abamectin and cyromazine. Some species are also controlled to a certain degree by spinosad.
Description and appearance
Leafminer adults are small, black and yellow flies. Liriomyza sativae is shiny black on the upper surface except for a prominent yellow triangle between the bases of the wings; the underside and the face between the eyes are yellow. Liriomyza trifolii differs in having the thorax covered with overlapping bristles that give fresh specimens a silvery gray colour; specimens that are carelessly handled or placed in alcohol lose the gray and appear black. Also, the portion of the head behind the eyes is mostly yellow in L. trifolii, with only a small black area touching the rear edge of the eye; in L. sativae, the area behind the eyes is predominantly black. Liriomyza huidobrensis adults are similar to L. trifolii, but slightly larger. With practice, field identification is possible. However, you may wish to contact your local farm advisor for verification. The yellowish maggots and the brown, seed-like pupae of the three species are too similar to distinguish in the field.
The leafminers Liriomyza sativae and L. trifolii are common. Both species can reach damaging levels quite rapidly if certain disruptive insecticides are used repeatedly. There has been a recent change in the pest status of a related species, L. huidobrensis, which has suddenly become dominant on other vegetable crops grown in coastal regions, and it appears to be spreading southward.
The three leafminer species are similar in life history. Eggs are inserted in leaves and larvae feed between leaf surfaces, creating a meandering track or "mine." At high population levels, entire leaves may be covered with mines. Mature larvae leave the mines, dropping to the ground to pupate. The life cycle takes only 2 weeks in warm weather; there are seven to ten generations a year. All three species feed on a wide variety of crops and weeds; development continues all year and the population moves from one host to another as new host plants become available.
Damage to the crop
Leafminer feeding results in serpentine mines (slender, white, winding trails); heavily mined leaflets have large whitish blotches. Leaves injured by leafminers drop prematurely; heavily infested plants may lose most of their leaves. If it occurs early in the fruiting period, defoliation can reduce yield and fruit size and expose fruit to sunburn. Pole tomatoes, which have a long fruiting period, are more vulnerable than other tomato crops. Leafminers are normally a pest of late summer tomatoes and can reach high numbers.
The presence of unsightly leaf damage in ornamental plants can lead to reduced market value.
Prevention
- Remove with pruners any isolated leaves with newly formed mines to stop a new leaf miner infestation before it can become serious. Monitor plants closely for the appearance of new mines. Wait for the appearance of one to three mines on each leaf before considering insecticides.
- Place plastic trays beneath the foliage of the affected tomato plants and check them daily for bright yellow leaf miner pupae. Kill those found by hand, but wait to apply an insecticide until you see 10 pupae appear daily over three to four days.
- Apply spinosad as a drench to the soil beneath the tomato's foliage to crops that are organically grown. Continue to monitor for new pupae using the plastic trays, and reapply spinosad every seven to 10 days until few or no pupae appear.
- Use a rotation of abamectin and cyromazine on the foliage of plants being produced using traditional chemical inputs. Apply abamectin first, and then wait two to three weeks to apply cyromazine. Repeat this rotation throughout the season, allowing two to three weeks between applications, until you no longer see leaf miners.
- Minimize leaf miner populations for the following year by plowing under spent tomato plants immediately. Rototill the garden thoroughly to ensure that pupae are buried deep underground.
Management
The most important aspect of leafminer management is conserving their natural enemies, which are often killed by broad-spectrum insecticides applied for other tomato pests. Reduce the risk of leafminer outbreaks by applying insecticides for fruit pests only when monitoring shows treatment is needed and by choosing insecticides that will not destroy the leafminer parasites.
Biological Control
Several species of parasitic wasps, particularly Chrysocharis parksi and Diglyphus begini, attack leafminer larvae; left undisturbed, parasites often keep leafminers under control.
Cultural Control
Check transplants before planting and destroy any that are infested; leafminers reach damaging levels earlier when infestations begin on seedlings. Tomato varieties with curled leaves, such as VF145s, are less susceptible to leafminer damage and may provide suitable alternatives where leafminer damage is expected, as in fields adjacent to other infested crops. Where a series of tomato crops is planted in the same area, you can reduce early infestations in a new crop by removing old plantings immediately after the last harvest.
Organically Acceptable Methods
Biological and cultural controls as well as sprays of the Entrust formulation of spinosad are acceptable for use on an organically certified crop.
Monitoring and Treatment Decisions
A monitoring technique for leafminers in fresh market tomatoes is to place plastic trays about 12 by 15 inches in size beneath plants at several randomly chosen places in the field. Mature larvae that drop from foliage accumulate on the trays and pupate there, providing a measure of leafminer activity. A treatment threshold used experimentally for L. sativae and L. trifolii in southern coastal fresh market tomato fields is to treat when an average of 10 pupae per tray per day accumulates over a 3- or 4-day period. In all areas, do not treat unless pupae are present. Absence of pupae, even if new mines are present, indicates that natural controls are keeping leafminers controlled.
The dominant species of Liriomyza leafminers is in flux. All species, however, have high resistance to organophosphates, carbamates, and pyrethroids. If these types of insecticides are used, leafminer populations will increase. Treatment recommendations currently involve the rotation of abamectin and cyromazine. Some species are also controlled to a certain degree by spinosad.
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