Oystershell scale - Mortality

Little is known about the mortality of oystershell scale in New Zealand. The data available relates to parasitism by wasps of the familiy Aphelinidae. Very high parasitism is reported in oystershell scale populations overseas. A study in Canterbury from July to December 1960 showed the highest parasitism in the second instar stage, with an average of 40% over the period. Peak losses occurred in October/November at 85-100% (see Table). A few adult scale insects were parasitised from late September, reaching 17% in November. No parasitism was found in prepupal and pupal males and only one instance in a first instar scale. The most common parasitoid species was Encarsia citrina. Predation of oystershell scale was also recorded, mainly by a camerobiid mite, Neophyllobius sp., assisted by whirligig mites (Anystidae). A study in Otago (1981-84) recorded parasitism by E. citrina (at lower levels than in Canterbury) and Aphytis mytilaspidis. High mortality of oystershell scale probably occurs during the aerial dispersal of crawlers, when only a small proportion would be expected to locate a suitable host plant. This will be countered to some degree by its wide host range.