Citrus Leafminer

Citrus leafminer larvae feed by creating shallow tunnels, or mines, in young leaves of citrus trees. The pest is most commonly found on citrus (oranges, mandarins, lemons, limes, grapefruit, and other varieties) and closely related plants (kumquat and calamondin).

Other mining-type pests (including a citrus peelminer that attacks the fruit and stems of citrus) attack weeds, ornamentals, and crop plants, but citrus leafminer is the only mining insect that commonly attacks citrus leaves. Citrus leafminer, Phyllocnistis citrella, was not found in California until 2000 when it was first detected in Imperial County. It soon spread to adjacent counties and continued to move northward.

Citrus leafminer now infests most of southern California, the coast as far north as San Luis Obispo County, and the San Joaquin Valley.The citrus leafminer is native to Asia. In the 1940s, it was first noted as a pest in Australia, and in the 1970s it showed up in other major citrus growing areas of the world. It arrived in Florida in 1993 and began making its way westward, invading northern Mexico in the mid-1990s and finally California.

Identification

Citrus leafminer is a very small, light-colored moth, less than 1/4 inch long. It has silvery and white iridescent forewings with brown and white markings and a distinct black spot on each wing tip. The hind wings and body are white, with long fringe scales extending from the hindwing margins.

The larval stage is found only inside mines of citrus leaves and other closely related plants. As it feeds and develops, the larva leaves a frass (feces) trail, observed as a thin dark line, inside the meandering serpentine mine just under the surface of the leaf. This visual characteristic is used to help identify the pest.

In its last stage the larva emerges from the mine and moves to the edge of the leaf. It rolls the leaf around itself and pupates in preparation for adulthood, creating a rolled and distorted leaf.

The citrus peelminer, a similar moth that attacks citrus, differs from citrus leafminer because its larval stages do not leave a frass trail in the mine, and it attacks fruit and stems rather than leaves. The peelminer pupa is wrapped in a silken cocoon covered with whitish, crystalline ball-shaped structures. Peelminer pupae are usually concealed in bark cracks and crevices and can be located anywhere on the tree.

Life Cycle

Citrus leafminer has four life stages: egg, larva, pupa, and the adult moth. Adults do not damage plants and live only 1 to 2 weeks. Adult moths are most active in the morning and the evening and spend the day resting on the undersides of leaves, but are rarely seen. Soon after emerging from the pupal case, the female moth emits a sex pheromone that attracts males. After mating, the female lays single eggs on the underside of host leaves. On the tree, the newly emerged leaflets of flush growth, particularly along the midvein, are the preferred egg-laying (oviposition) sites.

Eggs hatch about 1 week after being laid. The newly emerged larvae immediately begin feeding in the leaf and initially produce tiny, nearly invisible, mines. As the larva grows, its serpentine path of mines becomes more noticeable. The larvae molt 4 times over a 2- to 3-week period as they develop.

The larva emerges from the mine as a prepupa and rolls the edge of the leaf over causing a curling of the leaf. Inside that curled leaf edge the leafminer becomes a pupa. The pupal stage lasts from 1 to 3 weeks.

The entire life cycle of the insect takes 3 to 7 weeks to complete. Citrus leafminer develops best at temperatures between 70º to 85ºF and greater than 60% relative humidity, but will readily adapt to most California conditions.

Damage

Citrus leafminer can survive as a larva only in the tender, young, shiny leaf flush of citrus and closely related species. Older leaves that have hardened off are not susceptible unless extremely high populations are present. The larvae mine inside the lower or upper surface of newly emerging leaves, causing them to curl and look distorted.

Mature trees (more than 4 years old) that have a dense canopy of older foliage to sustain them can tolerate damage on new leaves during part of the growing season with negligible effect on tree growth and fruit yield. Very young trees do not have much mature foliage and they produce more flush year-round, thereby supporting larger citrus leafminer populations. Young trees may experience a reduction in growth. However, even young trees with heavy leafminer populations are unlikely to die.

Summer heat in the inland areas of California seems to suppress leafminer populations, but in cooler coastal areas, the insect population may remain high from summer through fall. The flush growth of citrus trees attacked by leafminer will look unsightly, but the best course of action is to leave it alone and let the natural enemies of the citrus leafminer feed on and parasitize the larvae in the mines, rather than trying to control this pest with insecticides.

In other areas of the world where the citrus leafminer invasion is long established, the experience has been similar: a high level of damage to citrus in the first year or two is followed by decreasing severity due to natural enemies parasitizing or consuming leafminers. These natural enemies, which are already present in the environment, survive by seeking out mining insects in which to lay their eggs. Eventually, the leafminer populations decline as the population of natural enemies increases. See the Biological Control section for more information.