L-2471

Biology and Ecology of the Citrus Leafminer

The citrus leafminer (CLM), Phyllocnistis citrella Stainton (Lepidoptera: Gracillariidae), is a pest of citrus species and other Rutaceae in many parts of the world. CLM larvae form mines predominantly in leaves, but also in succulent stems and sometimes fruit. The larvae bore through the leaf epidermis, in-gesting the sap and producing a chlorotic leaf patch. CLM may prevent young leaves from expanding, causing them to remain twisted and curled. After the CLM have finished feed-ing, other insects such as aphids and mea-lybugs often continue feeding on the dam-aged area.

CLM lifecycle. Total generation time for CLM ranges from 13 to 52 days, depending on temperature. The lifestages are: adult, eggs, larvae (four instars, including prepupa) and pupae.

The adult CLM is a small moth (Fig. 1) about 2 mm long with a wingspan of about 4 mm. Moths have silvery and white iridescent forewings with brown and white markings, and a distinct black spot on each wing tip. The adults copulate 14 to 24 hours after emergence and lay their eggs soon after-wards. A single female may lay about 50 eggs during her lifetime. The adults are most ac-tive from dusk to early morning; they spend the daytime mostly resting on the undersides of leaves. Adults feed on nectar and live from 2 to 12 days.

CLM eggs look like tiny droplets of water (Fig. 2) measuring only 0.3 x 0. 2 mm. They are transparent when newly-laid, but become yellowish and opaque in 2 days. The eggs are laid singly on either surface of the leaf, but may also be laid on succulent stems. Newly-emerged leaflets, particularly along the midvein, are the preferred oviposition sites. Larvae emerge from the eggs after 2 to 10 days.

The larvae undergo four instars and total development time takes from 5 to 20 days. First instar larvae begin feeding immediately, forming nearly invisible mines. The larvae are 1 to 2 mm long, light green and difficult to detect. During the course of larval develop-ment, the mines become more visible and lar-val excrement forms a central trail within the mine (Fig.3). When larval feeding is com-pleted, the third instar usually forms a mine that is directed towards the leaf margin. There, it molts into the fourth instar or prepupa. The prepupa is pale and almost cylindrical, and while it no longer feeds, it remains quite active. It forms a silken co-coon within the mine. As the silk dries, the leaf curls over the pupal cell. The pupal stage (Fig. 4) takes 6 to 22 days. The pupa is yel-lowish- brown, turning darker with age. Just before adult emergence, the pupa uses a spine on its head to make an opening at the anterior of the chamber, and forces its body through.

Foliar and fruit damage can be extensive when populations of CLM are high. Because the larvae generally do not cross existing mines, the later stages of feeding result in characteristically serpentine patterns (Fig. 5). Although mining usually occurs on leaves, it also can occur on fruit—particularly grape-fruit (Fig. 6), greatly reducing the market value of produce.

Host range and geographic distribution. CLM attacks all citrus cultivars, many related species in the Rutaceae family, and also some ornamentals. Grapefruit, tangerine and pummelo are among the most susceptible hosts. However, CLM has also been reported on jasmine, mistletoe, some legumes, willow and anaqua, Ehretia anacua (Mier & Berland) Johnst.

The CLM is believed to have originated in India and southern China, where it is a ma-jor constraint to citrus production. In the last century it has spread to the Philippines, Ja-pan, the Middle East, Australia and parts of Western Africa. The movement into North- ern Africa, Central America and the United States is very recent. CLM was first reported in the U.S. in Florida in 1993. By late 1994, the invasion had spread to Alabama, Louisi-ana and Texas. CLM was first found in Texas orchards (eastern Hidalgo County) in late August of 1994; by late November, it had spread throughout the South Texas citrus growing area.

Controlling the Citrus Leafminer

Chemical control. Insecticidal control is difficult to achieve because the larvae are shielded within the mines by the leaf epider-mis. The pupal stage is also protected by the rolled leaf margins. Moreover, the continu-ous use of pesticides induces resistance in CLM and is detrimental to its natural en-emies. Foliar sprays may be applied to pro-tect new flushes of growth when the leaves are most vulnerable to CLM damage. How-ever, the best foliar insecticides confer only 2 to 3 weeks of leafminer control, and insec-ticides cannot be relied upon for effective control of heavy leafminer infestations. Pe-troleum oil sprays applied to flush leaves may inhibit egg laying, but must be repeated ev-ery 14 days during each flush cycle. Chemi-cals are being tested and utilized for short-term CLM control. Eclipse® (fenoxycarb) was registered in early 1995 for use on nursery and nonbearing citrus. Research is empha-sizing the development of a long-term CLM management program using cultural and bio-logical controls. For current insecticide and dosage recommendations, see the latest Cit-rus Pest Control Guide obtainable from the Texas A&M-Kingsville Citrus Center in Weslaco.

Cultural control. Cultural control of CLM includes protecting the young plants (from 1 to 3 years old) and nursery plants. In ad-dition, it is important to limit flush growth when CLM population levels are high (usu-ally during summer and autumn) and to pro-mote flushing when populations are low. Flushing is limited by reducing fertilization and irrigation to the minimum required for normal plant growth. Mechanical hedging also should be delayed, if possible, until cooler winter temperatures prevail in order to limit regrowth. CLM infestations are low-est during the spring; the spring flush is pro-moted by fertilizing during the previous win-ter. These control methods may not be appli-cable in areas without pronounced seasons.

Natural enemies. The natural enemies of an insect fall into three general categories:

Parasites usually lay their eggs in or on the body of the host insect. The developing parasites feed on the host, eventually caus-ing its death.

Several indigenous species that attack CLM larvae and pupae in south Texas ap-pear promising. The most abundant species is the Eulophid Zagrammosoma multilineatum (Ashmead) (Fig. 7). Other Eulophid species are: Closterocerus cinctipennis Ashmead, Closterocerus sp., Neochrysocharis sp., Horismenus sp., and Pnigalio sp. One spe-cies from the Proctotrupoidea also was iden-tified as a parasite of CLM.

In Thailand, a number of parasites have been found for CLM, mostly Chalcidoidea. Ten species of chalcidoid parasites have been reared from the CLM in Thailand, with Ageniaspis sp. probably being the most sig-nificant. Total parasitism ranged from 25.4 to 91.9 percent in a pummelo orchard from May to September 1990.

In the United States, native parasites in south Florida cause parasitism rates in ex-cess of 50 percent in unsprayed citrus groves. Three of the most effective parasites in Asia (Citrostichus phyllocnistoides, Cirrospilus quadristriatus and Ageniaspis citricola) were introduced into Australia. The parasites A. citricola and C. quadristriatus were subse-quently collected from Australia and released into citrus groves in Florida in 1994. Prelimi-nary surveys indicate that A. citricola had established in 3 of 12 release sites, causing up to 50 percent parasitism. No evidence was found to indicate that C. quadristriatus has established. Ageniaspis citricola (imported through Florida) was released in Texas A&M University-Kingsville Citrus Center orchards in early 1995, but has not been established.

Predators feed on the pest insect. In many predatory insects, only the immature stages feed on other insects. The adults often feed on nectar. Several predators are known to reduce population levels of CLM, although they may have to be used in large numbers and in conjunction with parasites. Among these predators are various spiders, flower bug, Orius insidiosus, ladybugs, fire ants and the lacewing, Chrysoperla rufilabris (Fig. 8). The lacewing, a common predator that feeds on a variety of insect pests, is sold as a com-mercial biological control agent.

Pathogens cause insect diseases. They may be bacteria, fungi or viruses. Many com-mercial bioinsecticides are formulated from pathogens. There are no known naturally-occurring pathogens of the citrus leafminer. When the larvae are inside the mines, they are protected from pathogens. The pupae are also afforded some degree of protection by the curled leaf margins.

Figure 1. Citrus leafminer adult, Phyllocnistis citrella.	Figure 2. Transparent and ovoid-shaped egg of citrus leafminer.
Figure 3. Young larva of citrus leafminer.	Figure 4. Pupa of citrus leafminer.
Figure 5. Serpentine patterns made by larvae of citrus leafminer.	Figure 6. Mines on grapefruit caused by citrus leafminer.
Figure 7. Zagrammosoma multilineatum, a native parasite of citrus leafminer in South Texas.	Figure 8. Green lacewing (Chrysoperla rufilabris), a predator of citrus leafminers.

The information given herein is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement by the Cooperative Extension Service is implied.

Educational programs conducted by the Texas Agricultural Extension Service are open to all people without regard to race, color, sex, disability, religion, age or national origin.

We acknowledge J.Villareal and R. Saldaña for technical assistance. Photographs were provided courtesy of the Texas A&M University-Kingsville Citrus Center and the Texas Agricultural Experiment Station.

Mention of a trademark, proprietary product or vendor does not constitute a guarantee or warranty of the product by the Texas A&M University-Kingsville Citrus Center and does not imply its recommendation to the exclusion of other products that may be suitable.

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Last modified: December 29, 1997 by Rudolf Bendixen