California Red Scale: Plant Damage and Control

California Red Scale (Aonidiella aurantii), also called red scale, red orange scale, and cochinella roja Australiana, is a major pest of citrus which also occurs on on host-plants belonging to at least 77 plant families. Walnuts, apples, avocado, banana, figs, grapes, mangoes, olives, and pears may be severely affected where citrus is grown nearby. California red scale occurs in all citrus-growing areas of the world. Infestations on young trees are especially damaging. Young fruit also may be shed. A toxic substance may be produced that causes damage in addition to that caused by the feeding process. The scale infests all portions of the plant but thrives on the leaves and fruits, and, to a lesser extent, on the twigs. There may be as many as 150 crawlers on each leaf, and nearly twice as many on fruit. Crawlers may be transported by wind over considerable distances. 1,3

Adult males are tiny, oblong, yellow-winged insects; they do not feed and live rarely more than a day and their longevity is significantly shortened by high temperature. Adult females have a round cover and the body underneath is light brown, golden, or reddish. At high densities, the scale may defoliate portions of a tree, inhibit fruit production, and kill branches, and even sections of a tree. There are usually 2 to 3 generations a year, depending on the climate. All of its stages overwinter.1,2 Picture of California Red Scale

Mature females give birth to fully developed crawlers. The crawler its stylets into plant tissues, secretes a waxy scale cover, and becomes immobile. The life cycle from egg to adult takes about 6 weeks in summer.

Biological and Chemical Control

The easiest way to spot California Red Scale is to look for scales on fruit. Parasitic wasps are important to control red scale. Aphytis melinus, the most widespread natural enemy of the California red scale, has been reported to be one of the most important one. Approximately 5,200 parasitoids are released per acre every 2 weeks from mid-February to mid-August to provide consistent reduction of red scale in the San Joaquin Valley. The adult is a tiny yellow wasp which deposits eggs in immature scales. The parasitoid short life cycle (10-20 days) results in two to three parasitoid generations for each scale generation. Adult parasitoids are black, with two white stripes over the female's head.4



Recent research studies show that the parasitoids species prefer different scale instars and different periods of the year for parasitism. In the groves where A. melinus was present in high numbers, either alone or coexisting with Aphytis chrysomphali parasitic wasp species, parasitism rates may reach 22.4% on average, whereas parasitism rates were much lower (11.5%) when A. chrysomphali species was the dominant parasitoid. The best parasitism level was achieved when these ectoparasitoids occurred together with the endoparasitoid Encarsia perniciosi. The average annual mortality rate of A. aurantii may reach 73.6%.6

Oils (1:60), soaps, and microbial sprays are effective. Minimize dust by watering paths. Control ants, particularly the Argentine ant in southern California and the native gray ant in the San Joaquin Valley, because they disrupt red scale parasites. Excessive dust that coats the leaves and fruit, including dust from manure mulches as well as whitewash and kaolin clays interferes with parasitism and should be minimized or delayed until the end of the season when Aphytis has completed its work.1,5 Ant populations, especially those of Southern fire ant and Native gray ant, should be controlled before the release of parasitoids as they interfere with the parasitoid life cycle.

References

  1. Pests of the Garden and Small Farm: A Grower's Guide to Using Less Pesticide, Second Edition Mary Louise Flint
  2. Biological control in the Western United States: accomplishments and benefits of Regional Research Project W-84, 1964-1989 James Robert Nechols
  3. Armored scale insect pests of trees and shrubs (Hemiptera : Diaspididae) By Douglass R. Miller, John A. Davidson
  4. Tropical fruit pests and pollinators: biology, economic importance, natural enemies, and control Jorge E. Peña, Jennifer L. Sharp, M. Wysoki
  5. http://ipm.ucdavis.edu/PMG/r107301111.html
  6. Comparative efficacy of different combinations of natural enemies for the biological control of California red scale in citrus groves. Juan Sorribas, a, and Ferran Garcia-María




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