Better Yields Through Science

Citrus Diseases & Pests

DiseaseSymptoms, Life-cycle and pest management
Black ScaleScientific Name: Saissetia oleae

Female black scales reproduce without mating and lay 1,000 to 2,000 eggs over a period of 2 to 3 months, mainly during May and June and again from October through November in areas with two broods per year (cooler coastal regions). Crawlers move about for some time before settling on leaves. Feeding by black scale reduces tree vigor and can cause leaf or fruit drop and twig dieback. Excreted honeydew supports the growth of sooty mold. October to January is the right time for control with Oil and Lime sulfur.
Citricola ScaleScientific Name: Coccus pseudomagnoliarum

Citricola scale is a soft scale. Crawlers of the citricola scale appear from June through August. They settle primarily on the underside of leaves, but in severe infestations they also settle on the upper leaf surface and on twigs, rarely on fruit. Young scales are flat and almost translucent; they grow slowly over the course of the summer and fall, molting only once during that period. By November, immature scales turn a mottled dark brown color and begin migrating to twigs; this migration peaks in February and March. Once on twigs, they develop faster than they did on leaves and they turn a gray color. By late April, citricola scales molt and mature into the adult female stage. Females lay 1,000 to 1,500 eggs during the time from early May to early August. Eggs hatch after 2 to 3 days and crawlers move to leaves. There is only one generation a year, and there are no males.

Citricola scale may reduce tree vigor, kill twigs, and reduce flowering and fruit set. As they feed, citricola scale excretes honeydew, which accumulates on leaves and fruit. Sooty mold grows on honeydew and interferes with photosynthesis in leaves and causes fruit to be downgraded in quality during packing.
Citrus ThripsScientific Name: Scirtothrips citri

Adult citrus thrips are small, orange-yellow insects with fringed wings. During spring and summer, female’s lay about 25 eggs in new leaf tissue, young fruit, or green twigs; in fall, overwintering eggs are laid mostly in the last growth flush of the season. Overwintered eggs hatch in March about the time of the new spring growth. First-instar larvae are very small, whereas second-instar larvae are about the size of adults, spindle-shaped, and wingless. They feed actively on tender leaves and fruit, especially under the sepals of young fruit. Third- and fourth-instar (propupa and pupa) thrips do not feed and complete development on the ground or in the crevices of trees. When adults emerge, they move actively around the tree foliage.

Citrus thrips do not develop below 58°F (14°C). They can produce up to eight generations during the year if the weather is favorable. Shortly after petal fall, immature flower thrips can be seen moving around young fruit, but they soon pupate and adults disperse to other plants, consequently they are only concentrated in citrus orchards for a short period in spring.

On fruit, the citrus thrips punctures epidermal cells, leaving scabby, grayish or silvery scars on the rind. Second-instar larvae do the most damage because they feed mainly under the sepals of young fruit and are larger than first instars. As fruit grow, damaged rind tissue moves outward from beneath the sepals as a conspicuous ring of scarred tissue. Fruit are most susceptible to scarring from shortly after petal fall until they are about 1.5 inch (3.7 cm) in diameter.
Red Spider MiteScientific Name: Panonychus citri

Adult female citrus red mites are oval and globular; the male is smaller and has a tapered abdomen. Each female lays 20 to 50 eggs at a rate of 2 to 3 a day, depositing them on both sides of leaves. The life cycle from egg to egg may be as short as 12 days during warm weather. Populations increase in spring, late summer, and early fall in response to new growth; citrus red mites prefer to feed on fully expanded young leaves but will also infest fruit.

On leaves, citrus red mite feeding results in a pale stippling visible primarily on the upper surface of the leaf. In severe infestations, the stippling enlarges to dry necrotic areas (commonly called mesophyll collapse). Eventually, leaves may drop and twigs dieback. Stippling or silvering also occurs on green fruit but usually disappears when fruit change color. If large populations feed on nearly mature fruit, the silvering may persist. High populations can also cause fruit sunburn if hot weather is occurring. During fall, low levels of citrus red mite can cause a blasting or burning of foliage and leaf drop. Mites increase their reproduction on water-stressed trees. Good irrigation and limiting road dust reduces red mite outbreaks.
Rust Mite (Silver Mite)Scientific Name: Phyllocoptruta oleivora

This pest is known as the rust mite on oranges and the silver mite on lemons. Citrus rust mite is about the same size as a bud mite and requires a hand lens to view; it is deeper yellow in color than the bud mite and wedge shaped. A generation may be completed in 1 to 2 weeks in summer, but development slows or stops in winter, depending on temperature.

The rust mite feeds on the outside exposed surface of fruit that is 0.5 inch (1.3 cm) or larger. Feeding destroys rind cells and the surface becomes silvery on lemons, rust brown on mature oranges, or black on green oranges. Rust mite damage is similar to broad mite damage, except that somewhat larger fruit are affected. Most rust mite damage occurs from late spring to late summer.

Citrus rust mite tends to occur together with Broad Mite but usually in greater numbers. Both species thrive in warm, humid conditions. Monitor rust mite from early spring through summer. On orange trees, look for rust mites on young foliage in early spring; by late spring, most of the population will be on fruit. On lemon, rust mites are mostly on fruit throughout the season. If the population increases quickly or if scarring appears, a treatment is generally required. In some cases, the infestation is localized and a spot treatment may be sufficient for control.