The red scale and its insect enemies

Abstract

The red scale, Aonidiella aurantii (Mask.), was first noted in California in 1877. Two years later, introduction of its natural enemies was first suggested, and since 1891 nearly all the citrus-growing regions of the world have been searched for parasites or predators that would control it. At various times and in various countries claims have been made that parasites or predators or a combination of the two controlled red scale. True, there are countries, particularly China, where red scale seldom injures citrus seriously, or even rarely infests this host. This has been attributed sometimes to one, sometimes to another, or to a combination, of its insect enemies; but the evidence is not sufficiently convincing that its innocuousness in such regions is due solely to these enemies. All the factors need to be weighed, especially host-plant relations of red scale, which are complex and often contradictory from one country to another. The red scale is not a serious citrus pest in Florida, where insect enemies are not considered to be a factor, except by the most partisan adherents of biological control. All or nearly all of the insects claimed to control red scale anywhere have been imported into California, and some of them have become established; yet the red scale continues to rate as the most important scale pest of citrus in this state. In some seasons and in some limited areas a high percentage of the scales may be destroyed by parasites or predators, but at least up to 1948 the growers usually found it necessary to resume chemical means of control to protect the fruits and save the trees from injury. It is too early to appraise the results of some work done since 1948. Red scale was known as a pest of citrus in Australia much earlier than in California, and it may have been in South Africa even by 1857. It is a serious citrus pest in both countries, and attempts to find a biological control for it have followed a course similar to that in California. The consensus of Australian workers is that the scale is not controlled by its insect enemies there. Recently, on the other hand, South African workers have claimed that it is controlled in that country by a combination of parasites and predators if chemical control is omitted. In the long search for enemies of red scale, much effort has been wasted because of inadequate systematics. Up to 1937 a misleading classification of red scale and a failure to find any structural difference between it and two of its close relatives caused confusion. Further confusion and repetitious effort has been caused by similar trouble in the systematics of its insect enemies. In assaying the chances of finding a natural enemy to control red scale in citrus groves, it must be borne in mind that horticultural varieties of citrus differ radically from the original wild species and are grown commercially under conditions different from the humid tropics where the wild species are native.