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.
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.
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.
On mature backyard citrus trees, citrus leafminer rarely causes serious damage and management is normally limited to practices that limit succulent growth and protect natural enemies. Very young trees are more vulnerable to injury and insecticide treatment may occasionally be justified. However, available insecticides for backyard trees are not very effective and many products leave residues that kill natural enemies, compounding problems. Pheromone traps are available for detecting leafminer moths.
PHEROMONE TRAP and LURE SUPPLIERS Available in 2006 #
ISCA Technologies, Inc.
2060 Chicago Avenue, #C2
Riverside, CA 92507
Tel. (951) 686-5008
2828 SW Corbett Avenue, Suite 114
Portland, OR 97201
Tel. (877) 244-9610
Availability of pheromone trap producers is subject to change. Still under observation is the performance between different types of lures. No endorsement of named products is intended, nor is criticism implied of similar products not mentioned.
Traps baited with a pheromone (insect sex attractant) are a useful tool for detecting leafminers, determining when moths are flying and depositing eggs, and timing insecticide applications. However, they do not catch enough of the population to be used for control. Only male moths are drawn to the pheromone and become caught in the trap’s sticky coating.
Hang pheromone traps about shoulder height on a citrus tree. Follow the manufacturer’s recommendations for maintaining the trap, such as how often the pheromone dispenser should be replaced. Commercial traps specific to citrus leafminer will attract only citrus leafminer, but other insects may accidentally be caught in the trap.
Identify trapped moths before you decide to take a control action. Check traps every week for moths. However, the most important times to check are when the moth is most abundant and citrus is flushing in summer and fall. You may take traps containing moths to your University of California Cooperative Extension office or your county agricultural commissioner’s office for identification. Save identified moths for comparison when additional moths are captured later in the season.
Biological Control #
Citrus leafminers are killed by various parasites and predators, including tiny nonstinging, naturally occurring wasps such as Cirrospilusand Pnigalio species. The parasites lay their eggs inside the mine, inside or on top of the leafminer larva. When the parasite egg hatches the parasite larva consumes the leafminer larva. These parasites are very important for reducing citrus leafminer levels. Do not spray citrus with broad-spectrum insecticides and avoid other practices that disrupt natural enemies whenever possible.
Using physical controls and preventive cultural methods will also encourage the activities of native natural enemies. You do not need to release parasites, because many native parasite species that attack other leafminer species will also find your citrus trees and attack citrus leafminer.
Cultural Control #
Citrus leafminer moths are attracted to the new flush of citrus trees. Avoid pruning live branches more than once a year, so that the cycles of flushing are uniform and short. Once the leaves harden, the pest will not be able to mine the leaves. Do not prune off leaves damaged by citrus leafminer since undamaged areas of leaves continue to produce food for the tree. Do not apply nitrogen fertilizer at times of the year when leafminer populations are high and flush growth will be severely damaged, such as in the summer and fall.
Physical Control #
Vigorous shoots known as water sprouts often develop on branches and above the graft union on the trunk of mature trees. These shoots grow rapidly and produce new leaves for a prolonged period of time. Where citrus leafminer is a problem, remove water sprouts that might act as a site for the moths to lay eggs (oviposition). Suckers, the vigorous shoots which grow from the trunk below the graft union, should always be removed since they originate from the root stock and do not usually produce desirable fruit.
Chemical Control #
Many insecticides registered for residential use do not effectively control citrus leafminer, because they have difficulty reaching the larvae inside the mines. Leafminer infestations on mature trees rarely if ever require insecticide treatment. If very young or high-value trees are infested, insecticides can be applied to the new foliage when egglaying moths are active or to the soil in advance of new flush growth.
Insecticide products that contain the natural insecticides azadirachtin or spinosad show some efficacy against larvae and are safe for natural enemies. However, the residues do not last very long, and these insecticides might need to be reapplied every 7 to 14 days. Application of Green Light Spinosad is limited to 6 times per season.
Imidacloprid (Bayer Advanced Fruit, Citrus, and Vegetable Insect Control) applied to the ground at the base of citrus trees provides the longest period of control, 1 to 3 months. Imidacloprid should only be applied once a year. Imidacloprid applications should be timed to protect periods of leaf flushing, such as in the spring and fall. Imidacloprid takes 1 to 2 weeks to move from the roots to the leaves, so it should be applied as soon as new flush begins to appear. To protect bees, avoid applying imidacloprid during the period 1 month prior to or during bloom. Removing blossoms before they open on young trees will prevent honey bee exposure to imidacloprid in the nectar/pollen.
Frequent use of broad-spectrum insecticides such as malathion, carbaryl*, and pyrethroids are not recommended as they will kill beneficial insects and can result in a buildup of whiteflies, scale insects, and other citrus pests.
*As of August 1, 2020, pesticides containing the active ingredient carbaryl are restricted use materials in California. A valid pesticide applicator’s license is required for their possession and use. For more information see the California Department of Pesticide Regulation website.