Biological control of the San Jose scale Quadraspidiotus perniciosus (Comstock) (Homoptera: Diaspididae) in southern California.
The distribution of Quadraspidiotus perniciosus (Comst.), both geographically throughout southern California, and on different food-plants, is reviewed; it seldom becomes a serious pest of any one species of economic plant, and then it usually infests only part of the plant. From its wide geographical and climatic distribution it could be concluded that weather did not appreciably regulate populations. The only 3 natural enemies of any importance in southern California were the parasitic Hymenoptera Aphytis aonidiae (Merc.) and Prospaltella perniciosi Tower, and the parasitic mite Hemisarcoptes malus (Shimer); the last 2 species appeared to be limited to areas with coastal climatic influences, but A. aonidiae was the most common and widespread and was dominant in all localities sampled. Climate affected the age-distribution of Q. perniciosus, and at the coldest of the localities sampled the synchrony between host and parasite was interrupted during the winter, which was however partly counteracted by the winter diapause of A. aonidiae. In the 4 milder localities, scales in stages suitable for parasite development were present all the year round. Total parasitism was never very high but was associated with high rates of mortality, of which an important one was probably host-feeding by adult females of A. aonidiae. The 3 parasite species mentioned were able to coexist and complement one another since they parasitised different stages or instars of the host. Paired-cage comparative tests on plots in fruit orchards where parasites were present or excluded proved that the generally low populations of Q. perniciosus in the field, in spite of the favourable climate of southern California, were due to parasite activity.