Leafminers

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 gives fresh specimens a silvery gray color; 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. With practice, field identification is possible. However, you may wish to contact your local farm advisor for verification. The yellowish maggots and the brown, seedlike pupae of the two species are too similar to distinguish in the field.

The leafminers Liriomyza sativae and L. trifolii are common throughout California. Both species can reach damaging levels quite rapidly if certain disruptive insecticides are used repeatedly. Liriomyza trifolii, which appeared in the state in the late 1970s to early 1980s, is resistant to a wide spectrum of pesticides and has been the most common leafminer pest of tomatoes since 1990.

The two 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. Both 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.

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 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 are least likely to harm leafminer parasites.

Several species of parasitic wasps, particularly Chrysocharis parksi and Diglyphus begini, attack leafminer larvae; left undisturbed, parasites often keep leafminers numbers below economic injury levels.

Check transplants for leafminers or mines before planting and destroy any plants that are infested; leafminers reach damaging levels earlier when infestations begin on transplants. Tomato varieties with curled leaves 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.

Biological and cultural controls as well as sprays of the Entrust formulation of spinosad are acceptable for use on an organically certified crop.

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 parasitic wasps are suppressing leafminer numbers.

The dominant species of Liriomyza leafminers in California is in flux. However, all species are resistant to organophosphates, carbamates, and pyrethroids. If these types of insecticides are used, Liriomyza leafminer numbers will increase. Rotate applications of abamectin (also controls russet mite) and chlorantraniliprole or spinetoram. Some species are also controlled to a certain degree by spinosad.