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Source: UC Integrated Pest Management (http://www.ipm.ucdavis.edu/PMG/r107303211.html)
DESCRIPTION OF THE PEST
Citrus leafminer is a very small, light colored moth that arrived in southern California from Mexico in 2000. Citrus leafminer has been moving northward in backyard and commercial citrus since that time and now infests citrus in southern and central California.
Adult citrus leafminers are tiny moths about 2 mm long (less than 0.12 inch) with a wingspan of about 4 mm (or about 0.25 inch). They have silvery and white iridescent forewings with brown and white markings and a distinct black spot on each wing tip. Moths are most active from dusk to early morning and spend the day resting on the undersides of leaves, but are rarely observed. Soon after emerging from the pupal case, the female emits a sex pheromone that attracts males. Females lay eggs singly on the underside of leaves. Newly emerged leaflets (flush), particularly along the midvein, are the preferred oviposition site.
Eggs hatch about 4-5 days after being laid and newly hatched larvae begin feeding immediately in shallow, meandering mines in the leaves. As a larva increases in size, the mine becomes more visible and larval excrement forms a thin, central frass trail within the mine. Larvae molt 4 times over a 1 to 3 week period. Mature larvae pupate within the mine, rolling the edge of the leaf and protecting the pupa with silk. The entire life cycle of the insect takes 2 to 7 weeks to complete, depending on temperature and weather conditions. The activities of citrus leafminer vary somewhat with location in the state because of differences in climatic conditions and flushing of citrus trees. In general, citrus leafminer is active from mid-summer through fall and early winter.
The citrus peelminer, a small moth that attacks citrus, differs from citrus leafminer because its larval stages do not leave a frass trail in the mine, and it attacks stems and fruit rather than new flush leaves. Also, the peelminer pupa has decorative balls on its cocoon whereas leafminer pupae are found in the curled edge of a leaf and lack decorative balls.
Citrus leafminer larvae feed by creating shallow tunnels, referred to as mines, in young leaves. It is most commonly found on citrus (oranges, mandarins, lemons, limes, grapefruit and other varieties) and closely related plants (kumquat and calamondin). The larvae mine the lower or upper surface of the leaves causing them to curl and look distorted. Mature citrus trees (more than 4 years old) generally tolerate leaf damage without any effect on tree growth or fruit yield. Citrus leafminer is likely to cause damage in nurseries and new plantings because the growth of young trees is retarded by leafminer infestations. However, even when infestations of citrus leafminer are heavy on young trees, trees are unlikely to die.
In Florida, citrus leafminer creates openings that allow for entry of citrus bacterial canker into the tree resulting in infection. Citrus bacterial canker is not found in California. Annual surveys for citrus bacterial canker are conducted by the State of California, and other regulations are in place to exclude its introduction.
Mature Citrus Orchards (more than 4 years old). While the new flush of mature trees may be heavily damaged by citrus leafminer and look unsightly, yield and tree growth of most varieties will be unaffected. Therefore, insecticide treatments are generally not needed for mature citrus orchards. The exception to this is coastal lemons, which have multiple growth flushes. Citrus leafminer damage weakens leaves, making them more susceptible to wind damage and other pests; studies are underway to determine if yield is also affected. Worldwide, citrus leafminer populations are fairly well controlled by parasitic wasps. However, citrus leafminer has only recently entered the state of California and parasites are not uniformly present or active in all regions where citrus leafminer has recently established. Whenever possible, do not spray citrus with broad-spectrum insecticides and avoid other practices that disrupt natural enemies whenever possible to encourage natural enemies. Citrus peelminer and leafminer share many of the same parasites including Cirrospilus and Pnigalio species.
Young Citrus Orchards (less than 4 years old) . Because citrus leafminer can retard the growth of young trees, apply insecticides to nursery citrus trees and new plantings of citrus. Imidacloprid (Admire or Nuprid) applied through the irrigation for young trees or to the soil of potted citrus provides the longest period of control (1 to 3 months). The length of control depends on tree spacing and soil and irrigation conditions. Time applications of Admire or Nuprid to protect periods of flushing.
Foliar insecticides suppress citrus leafminer for shorter periods of time (several weeks) compared to Admire or Nuprid. Foliar treatments are effective for only 2 to 3 weeks because citrus leafminer adults lay eggs on new flush growth that was not present at the time of treatment. Oil has been shown to work as a temporary oviposition deterrent in nursery settings but should be used with care to avoid phytotoxicity. Diflubenzuron (Micromite) is effective primarily against eggs and larval stages.
Citrus leafminer moths are attracted to 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 because undamaged areas of the 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.
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). Always remove suckers, the vigorous shoots that grow from the trunk below the graft union, because they originate from the rootstock and do not produce desirable fruit.
Traps baited with a pheromone (insect sex attractant) are available for citrus leafminer and are a useful tool for determining when moths are flying and depositing eggs. Hang a trap containing the pheromone inside a citrus tree at about chest height during March through November. Follow the manufacturer's recommendations for maintaining the trap, such as the frequency with which pheromones should be replaced. Use one pheromone trap per 5 acres. Check the traps weekly for moths. Citrus leafminer moths may be captured in traps almost any time during the growing season. However, this species is most abundant when citrus is flushing in the summer and fall months. These traps will help you determine when male flights are occurring and when to time insecticide applications if they are needed. Ovicides such as oil or diflubenzuron (Micromite) should be applied during peak flights of moths.