Unaspis citri (citrus snow scale)

Infestations of U. citri usually occur on the trunk and main limbs of trees under ten years old. Heavy infestations spread to the twigs, leaves and fruit. This results in yellow spotting on the undersides of leaves which drop prematurely, dieback of twigs and weakening and eventual killing of branches. Heavily infested bark becomes dark, dull, and hard, appears tight and subsequently splits. Weakened limbs and twigs become infected with fungi and may be subsequently attacked by wood-boring insects.

Regulatory Control (Plant Quarantine and Certification)

Importation of citrus plants for planting from countries where U. citri occurs should be prohibited. Fruits should be subject to requirements such as area freedom, place of production freedom or treatment.

Biological Control

Existing biological control agents include the hymenopterous parasitoids Aphytis lingnanensis used in Florida (USA), the Solomon Islands and Cuba, and Aspidiotiphagus lounsburyi in Cuba. Construction of field cages to enclose citrus trees has facilitated the release and establishment of A. lingnanensis in citrus groves in Florida (Brooks and Vitelli, 1976). U. citri increased greatly in numbers after 1963 in Florida and was not effectively suppressed by biological methods. High-volume pesticide sprays were required for control (Simanton, 1974). Browning (1994) provided an up-to-date assessment of biological control in Florida. Partial success has been obtained with A. lingnanensis and efforts are being made to make additional introductions.

Chemical Control

Chemical control is possible but the waxy surfaces, sessile nature, intermittent feeding and overlapping generations of U. citri make it difficult to control. In Cuba, the insecticides sulphur, carbaryl, dimethoate and malathion are commonly used (Castineiras and Obregon, 1986). In laboratory and field trials, mineral oil or a mineral oil/dimethoate mixture was found to be the most effective against the mobile stages and against the general population (Fernandez and Rodriquez, 1988).

U. citri is one of the principal pests of Citrus spp. in many of the citrus-growing regions of the world. It infests the trunk, branches and small shoots causing serious damage to orchards due to leaf drop and rapid dieback. Relatively small numbers of scales can cause damage.