Parasitism by Aphytis chrysomphali, Mercet and A. melinus Debach of Californian red scale, Aonidiella aurantoo (Maskell), in relation to seasonal availability of suitable stages of the scale

Abstract

A field study was undertaken to investigate the host-parasite relationship of A. aurantii and two species of Aphytis in north-western Victoria, and to suggest methods for improving and extending the biological control of A. aurantii in Victoria. Monthly samples of scale-infested Valencia oranges from five unsprayed biocontrol properties at Mildura provided information which showed a positive correlation between the percentage of available stages in the host population and the efficiency of parasitism by the Aphytis. This was thought to result from the seemingly inefficient method employed by the Aphytis in selecting a suitable host in which to oviposit; a postulate reinforced by the results of earlier work. The scarcity of suitable hosts at certain times of the year is thought to limit the efficiency of the Aphytis by reducing the chances of the parasite to achieve its reproductive potential. On the basis of this study an improved approach to red scale control is proffered, namely that attempts be made to augment the effectiveness of the established Aphytis by devising methods for increasing the proportion of available stages in red scale populations during late spring and early summer. Field and laboratory evidence indicates that parasitism by Aphytis, rather than variation in weather, is responsible for the considerable seasonal fluctuations in percentage of scales available for parasitism.