Biological studies onAphytis aonidiae(Mercet) (Hymenoptera: Aphelinidae), an important parasite of the San Jose scale

Abstract

San Jose scale is well distributed throughout southern California on a variety of host plants but is quite rare in general. Only occasional plants, and usually only a portion of each, have any appreciable numbers of scales. Examples of these plants in several climatic zones were studied. Historical evidence, wide geographical distribution, and deductive and inductive reasoning essentially ruled out the weather as having any appreciable regulatory effect on scale populations. Out of some six species of natural enemies that attack the San Jose scale in southern California, only three were found to be of any consequence—the hymenopterous parasites, Aphytis aonidiae (Mercet) (see Note, page 205) and Prospaltella perniciosi Tower, and the parasitic mite, Hemisarcoptes malus Shimer. Aphytis aonidiae is the most common and widespread, constituting nearly 80 percent of the specimens reared or counted. It was virtually the only species present in two of the five districts studied and was dominant in all. Prospaltella perniciosi and H. malus appeared limited to areas having coastal climatic influences; A. diaspidis was rare and inconsequential. Winter climate affected the age distribution of scale stages; and at the coldest location, host-parasite synchrony was interrupted during the winter. This was partially alleviated by overwintering diapause of A. aonidiae. The four milder locations had scale stages suitable for parasite development all year. Total percentage parasitization was never impressively high, yet it was always associated with much higher proportions of scales dead from cryptic causes. The majority of such deaths was ascribed to host-feeding by adult female A. aonidiae. Hemisarcoptes malus developed only on mature third-stage female scales, A. aonidiae only on second-instar and third stages, and P. perniciosi on second-instar, second molt, and third stages. Thus, these three can coexist and complement one another. These life-table studies may indicate, but do not in themselves prove, that parasites were regulatory at the densities observed in the field. Paired-cage comparisons between scale trends on plots having parasites vs. those with parasites excluded proved that the parasiteswere responsible for the generally low scale population densities observed in the field in southern California, and that climate was permissive for scale development. These tests showed that A. aonidiae, acting alone, can regulate San Jose scale populations at low levels. Based on these studies, we suggest that the potential of A. aonidiae for biological control of San Jose scale should be reexamined in all countries where this scale is a problem.