San Jose scale (SJS) have long been considered a secondary pest for apple growers as they do not attack the fruit directly, so the temptation over the years has been to minimize their potential to be a serious concern in most orchards.
However, relationships between pests and the crops they attack are rarely stable, and SJS is a good example of how they can emerge again as a significant challenge to healthy apple trees and fruit. Growers have seen a marked increase in infestations of what used to be considered a marginal insect pest over the last 10 to 15 years, largely because of changes in chemical control programs.
Asian Invader
Originally from China, San Jose scale, Quadraspidiotus perniciosus, was introduced into the U.S. (California) on infested plant stock in the 1870s. It is now cosmopolitan in its distribution, occurring nearly everywhere tree fruits are grown; it attacks principally pome fruits such as apple and pear, stone fruits — nectarine, peach, plum, and cherry — and such tree nuts as almond and walnut.
Frequent sources of infestation are nursery plants, or as wind-dispersed crawlers from both fruit and non-fruit hosts. Three principal factors favoring infestation by this pest are: the high number of potential host plants (700-plus species), a high reproduction rate, and an absence of effective natural controls in pesticide-based agroecosystems.
SJS mainly colonizes the wood tissue of branches and twigs, although it establishes on fruit surfaces when populations are high. Damage is caused by the feeding of the immature forms, “crawlers,” which suck plant sap, weaken the plant, reduce fruit and shoot growth, and desiccate foliage.
Infested trees usually exhibit less foliage and smaller fruits; a reddish “halo” surrounds the point of scale attachment on the fruit, under the skin, which is caused by the plant’s reaction to toxin in the saliva. Smooth-skinned fruits, such as apples and pears, are more susceptible than those with rough or velvety texture, like peaches.
There are two generations per growing season in the Northeast. They overwinter as immatures under scale covers called “black caps,” mature to adults in spring, and males emerge and mate around petal fall. Crawlers emerge about mid-June and in early August, an event that can be timed by using degree day (DD) accumulations — first generation: 310 DD (base 50°F) after first adult catch, second generation: 400 DD after first adult catch. It’s possible to monitor for crawlers using tape traps on scaffold branches.
