Leafminers

Adults are small black to gray flies with yellow markings. Females puncture leaves to feed on plant sap and lay eggs within the leaf tissues. After 2 to 4 days eggs hatch and larvae feed between the upper and lower surface of the leaves, making distinctive winding, whitish tunnels or mines that are often the first clue that leafminers are present. Larvae emerge from the mines and pupate on the leaf surface or, more commonly, in cracks in the soil. Many generations occur each year and the entire life cycle can be completed in less than 3 weeks when the weather is warm.

The species can be separated on the basis of the adult's appearance or by the appearance of the mines. The serpentine leafminer is most common in the warmer, celery-growing areas of southern California and the Central Valley; it does not occur in Central Coast growing areas. The pea leafminer occurs in the Salinas Valley as well as in the central coast growing areas where it occurs with the serpentine leafminer. It also appears to be extending its distribution into the southern growing regions of the state. The vegetable leafminer may be present in the coastal areas in low populations.

The serpentine leafminer is the smallest and most yellow of the three and produces characteristically wandering mines. The pea leafminer is the largest and most black or gray and, following the first two instars, produces straight mines. The pea leafminer is more likely to mine the petioles and stalks of celery than the other species.

Leafminers can reduce the plant's photosynthetic capacity, render edible portions of the plant unmarketable, and provide an entrance for disease organisms. In addition, many countries and some states regulate the importation of produce grown in areas where pea leafminer exists to prevent its introduction.

Natural enemies, especially parasitic wasps in the genus Diglyphus, commonly reduce populations of leafminers, unless killed off by insecticides applied to control other pests. Several other parasites also attack leafminers. Predators are not important biological control agents because leafminer eggs and larvae are protected within leaf tissue. Choose selective pesticides for treating other pests, if possible, to avoid killing parasites and inducing leafminer outbreaks.

Liriomyza leafminers attack a wide variety of vegetable crops often grown in proximity to celery. Where possible, avoid planting next to infested fields, especially lettuce fields near harvest.

Biological and cultural controls and sprays of azadirachtin (Neemix) are acceptable for use on organically grown produce.

Treatment thresholds must be based on the species involved. To measure infestation levels of the serpentine leafminer, place trays between the rows of celery to catch dropping pupae. Trays about 4 inches by 9 inches fit well between the rows. Catches of 15 pupa or more per day per tray may be indicative of populations that require treatment.

Populations of pea leafminer or vegetable leafminer are not as easily monitored, and the treatment threshold is probably considerably lower than for the serpentine leafminer because of the tendency of this species to mine the stalks.