Citrus red mite—Panonychus citri
This spider mite (family Tetranychidae) can discolor fruit and leaves of any citrus variety. It occurs in all citrus-growing areas of California but is most common in the Central Valley during late winter and spring. Although its discoloring damage can be dramatic, citrus can tolerate high populations without affecting fruit yield. An exception is during fall and winter Santa Ana conditions in Southern California. Then dry winds combined with moderate red mite densities can cause substantial premature drop of leaves that greatly stresses host plants.
Citrus red mite produces little or no webbing on plants. This distinguishes it from some other Tetranychidae in citrus, which excrete fine silk strands on foliage and fruit.
Citrus red mite adults and nymphs are red to purplish. Adult females are globular, oval, and about 1/50 inch (0.5 mm) long. The male is somewhat smaller and its abdomen tapers toward the rear. Males are commonly found near molting female deutonymphs (immatures), waiting to mate with them when they mature into adults.
Eggs can occur anywhere on citrus but are often found along the midrib vein of leaves. The globular, red eggs have a curved stalk on top and from its tip 10 to 12 threads radiate down to the leaf surface. Eggs of European red mite are also red with a stalk, but lack the radiating threads.
At least seven species of spider mites can be pests on citrus in California. Although citrus red mite is the most common of these, hydrangea mite (Kanzawa spider mite), Lewis spider mite, sixspotted mite, Texas citrus mite, twospotted spider mite, and Yuma spider mite can also discolor citrus fruit or leaves or both in certain growing areas.
Citrus red mite and other spider mites develop through 5 life stages. The egg hatches into a 6-legged larva, then becomes an 8-legged protonymph, deutonymph, and adult. Preceding each molt, mites enter a brief resting stage during which they do not move. The life cycle from egg to egg-laying adult can be as short as 12 days.
Each adult female citrus red mite lays from 20 to 50 eggs at a rate of 2 or 3 a day, depositing them on both sides of leaves. Citrus red mite overwinters primarily as eggs, although all stages can be present year-round. Populations increase in the spring, late summer, and early fall in response to new leaf growth; citrus red mites prefer to feed on young leaves but will also infest fully expanded mature leaves and fruit. In cooler growing areas on young lemon and orange trees, the mites can be numerous throughout the year because repeated growth flushes provide preferred food almost continuously. Natural enemies, temperatures above 90°F, low humidity, and the absence of succulent, young foliage reduce the abundance of citrus red mite.
Citrus red mite feeds on more than 100 plant species including almond, pear, and rose, but it is only a pest on citrus. Adults and nymphs suck cell sap from leaves and fruit, causing bleaching or pale stippling of surfaces. Stippling of the leaves or fruit peel does not hurt the quality of the fruit inside. In severe infestations, leaf stippling enlarges to become dry, necrotic (brown) areas. Severely damaged leaves may drop prematurely, and twigs may die back. In coastal and Southern California growing areas, during Santa Ana winds in the fall, citrus red mite in combination with the wind can result in blasting or burning and drop of leaves. High mite populations that cause leaf drop can also contribute to fruit sunburn and reduced fruit yield during the next crop.
The mite stippling or silvering of green fruit usually disappears as fruit develop their mature color. When abundant mites feed on nearly mature fruit, the silvering may persist, but this damage is only aesthetic.
Damage from citrus red mite in Southern California appears mostly in late summer or fall. In the San Joaquin Valley, mite damage is most prevalent in the spring. In the desert valleys, citrus red mite is an occasional problem in spring and a rare problem in the fall. Citrus red mite is more of a problem when trees are water stressed, conditions are hot and dry, and after broad-spectrum pesticides have been applied for any citrus pest.
Ensure that trees are adequately irrigated, especially when hot or windy weather is anticipated. Citrus red mites have numerous natural enemies, which often keep the pest from becoming abundant on trees that are not treated with miticides or broad-spectrum insecticides. Predaceous mites and insects and a virus disease can greatly reduce the abundance of citrus red mite.
Euseius species of mites are the most important predators of citrus red mite in most growing areas. Predatory mites feed mostly on eggs and immature stages of citrus red mite. The spider mite destroyer (a tiny black lady beetle, or ladybug) and sixspotted thrips are the most important mite predators in desert valleys, but in other growing areas they are less commonly observed in comparison with Euseius species. Green lacewings can be common predators in all growing areas. Predaceous mite midges (Feltiella species) and (less commonly) a rove beetle (Oligota oviformis) prey on citrus red mite in coastal areas.
A virus disease specific to citrus red mite is widespread in citrus-growing areas away from the coast. When hot, dry temperatures exceeding 90°F occur for several consecutive days and high mite populations are present, virus epidemics occur and rapidly reduce citrus red mite populations. Symptoms of virus infection include mites that walk stiffly and curl their legs under their body. As they die, their bodies disintegrate and sometimes leave reddish brown to black watery spots on fruit or leaves. If diseased mites are mounted on a slide and examined under a light polarizing microscope, virus crystals that shine can be observed in the cell nuclei.
To improve the effectiveness of biological control, control ants, minimize dustiness (e.g., periodically hose off small trees), and avoid the application of miticides and broad-spectrum, persistent insecticides for all citrus pests. See Protecting Natural Enemies and Pollinators for more suggestions.
Insecticide application is generally not necessary for residential citrus trees to protect fruit flavor or yield. If spider mites have been intolerable, a spray of horticultural oil in August or September in Southern California and coastal areas or spring sprays in the San Joaquin Valley can reduce mite abundance. Note that oil sprayed during hot weather can injure foliage. For more information, see Citrus Red Mite and the Predatory Mite Euseius tularensis (you must enroll at the website to view this) and Pest Notes: Spider Mites.
Adapted from Integrated Pest Management for Citrus, Pests of the Garden and Small Farm: A Grower's Guide to Using Less Pesticide, and Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).
Mite-bleached leaf (foreground) compared with undamaged leaf.
Tiny citrus red mites feeding on lemon rind.
Adult female citrus red mite.
Nymph (left) and eggs of citrus red mite.