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Abstract
INTRODUCTION
Citrus is considered to be one of the most important fruit crop in Egypt, because of its economic importance in exporting and local consumption. In addition, it represents more than 50% of the total planted area of fruit trees. The production of orange reached 2.251 million tons/year (2005) and export quantity of fresh orange is about 480,000 tons (FAO yearbook, 2006).
Citrus is attacked by a number of insect pests. Some of these insects infest leaves and suck the plant sap i.e. aphids, scale insects, mealy bugs and citrus whitefly. Other insects feed on the flowers or fruits, i.e. fruit fly and citrus flower moth. The small leaf mining moth, Phyllocnistis citrella Stainton, family Gracillariidae (subfamily Phyllocnistinae), or the citrus leaf miner (CLM), was found in Egypt in south Sinai 1993. The citrus leaf miner (CLM) is a major pest of citrus causing extensive damage to new leaf flush. More than 80% of the orange orchards in Egypt where already infested with this insect i.e. (Ismailiya, Sharkiya, Dakahliya, Gharbiya, Behaira, Alexandria, Qualubiya, Monofiya, Giza, Bani-suef, Fayoum, Elmenia, Assiuot and Sohag).
During CLM larval stages, larvae mine tender foliage, killing leaf tissue and causing leaf drop.
P. citrella mines the abiaxial surfaces of new leaves. Injured leaves curl, become chlorotic then necrotic. Consequently, heavily infested leaves (>4 mines per leaf) are frequently distorted and may abscise (Pena and Duncan, 1993). Characteristic silvery mines are produced during the feeding stage of the first larval instars. The silvery appearance is due to the air and condensed water vapor trapped in the mine. As the leaves mature, they are not liable to attack. In cases of massive infestation, CLM attacks even young fruits (Heppner, 1995).
Heavy infestations can retard growth of nursery and newly planted trees and may reduce yield of bearing trees. Yield reduction can reach up to 50% and fruit weight can be reduced from 120g to 70g (Knapp et al., 1994) and injury may also occur in succulent stems and fruits. Larvae pupate near the leaf edge. Small eggs are laid on young leaves and the hatching larvae produce serpentine mines beneath the leaf epidermis where they fed upon the liquid contents of leaf cells and some leaves drop, which ultimately results in a reduction in the tree’s photosynthetic capacity.
The combination of the rapid spread of CLM throughout the worldwide; heavy levels of infestation and damage being seen, implication in the spread of citrus canker has promoted this species to be one of the most feared pests in citrus production.
Larvae of P. citrella also attack tender twigs during severe infestations. Such injury can be severe causing great damage and consequently reducing tree vigor and productivity (Sabine, 1971). The pathogen in form of pustules is more abundant along the mines on the damaged leaves. There could also be an association between P.citrella and citrus canker.
A single larva can consume 1 to 7 cm2 of leaf area making a mine of 6 to 11.5 inch. Yield reduction in mature trees is uncommon, but has been reported in limes heavily infested with the citrus leaf miner. Mining of spring flush is more damaging than fall flush since the spring flush is primarily responsible for supporting fruit development. Trees of three years age or less are especially susceptible to leaf miner damage. Severe damage to young trees may result in delayed maturity of 1 to 2 years.
The availability of tender growth flushes appeared to be the most important factor