Author: Eric Day and Douglas G. Pfeiffer Virginia Tech
Apple, pear, peach, plum, Osage orange, or ornamental trees and shrubs
Description of Damage
San Jose scale is one of the most destructive scale insects. Scale insects feed on plant juices by means of long, hollow, needle-like mouthparts inserted into the tissue of the host. The host tissue near the feeding site often turns red or purple (this is most evident on bark by making a thin slice into the bark with a knife). When feeding on apple fruits, the small brown scale has a characteristic red halo around it. Slow progressive killing of infested twigs and stems often results. Heavy infestation may cause death of the tree. Damaging stages are nymphs and adult females. Plant parts attacked are fruits, twigs, stems, and branches.
The immature and adult females live under a small circular brown or smoky black cover up to 1/16th of an inch (1 to 2 mm) in diameter. The scale has dark concentric rings that can be seen under a magnifying glass. A few elliptical-shaped scales (larger male immatures) may be interspersed among the otherwise round scales. The San Jose Scale, Quadraspidiotus perniciosus (Comstock), is in the order Homoptera family Diaspididae.
San Jose scales start life as crawlers which are tiny, free- living insects that move about the plant looking for a place to settle. The crawlers can be moved to another plant by wind or by hitching a ride on a bird's leg. This is the only time that the scale can move from plant to plant. After the scale settles, it quickly starts to secrete its waxy covering and remains in place for the rest of its life. There are at least three generations per year in Virginia. The first crawler emergence is in late May through early June. The date of the first crawler appearance varies by up to two weeks from year to year and also between areas within a given year. The second generation crawlers occur in late July, with the third generation in late summer or early fall; but due to overlapping of generations there is some variability to exactly when it occurs. By the end of the season, generations are no longer discrete and all stages are present. Crawlers usually settle very close to where they hatched and heavily infested plants have bark crusted with scales. If the crawler is a male, it will develop like the female early in its life, but during the later stages of development the male scales form what is known as a pupal stage from which emerges a tiny two-winged insect capable of flight. After mating with the female, the male dies. Adult females remain under cover and lay eggs. Crawlers appear about 50 days after the first male flight. Immature male and female scales overwinter and mature in the spring.
Apply superior dormant oil in late winter for the overwintering stages. During the growing season, treat when crawlers are first seen and again 7-10 days later. To observe crawlers, wrap black electrical tape around a scale infested branch with the sticky side outward. Check each day for lemon-yellow crawlers caught on the tape. When the tape losses its tackiness, replace it. Although variable, the crawler dates in Virginia are usually about June 10-15 and late July. See the Virginia Pest Management Guides for specific insecticides recommended for control.
Females produce a sex pheromone to attract the winged males. Synthetic pheromones have shown some possibilities for monitoring and control. If pheromone traps are used for monitoring, expect crawler emergence about one month after the first male catch (traps should be hung at bloom or shortly before). Sprays directed toward male flight can be effective as crawler sprays, but often male activity occurs during bloom when sprays may not be applied. Insecticides should not be used at bloom because this will disrupt pollination by bees.