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San Jose scale - Mortality |
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There have been few studies of the causes of mortality to San Jose scale populations in New Zealand. The little quantitative data available mainly relates to levels of parasitism by wasps of the familiy Aphelinidae. A study of San Jose scale on apples in Nelson from 1968-70 found 20-88% were parasitised (see Table). Highest levels occurred in autumn, winter and spring, with lower levels in mid-summer. The scale population on the young wood of the trees suffered higher parasitism than the population on the bark of older branches. In this same study, fungi were reported to cause up to 50% mortality in some seasons. All the parasitoids shown in the Table were caught in San Jose scale pheromone traps at the same Nelson site from 1978 to the early 1980s. The dominant species trapped was Encarsia perniciosi. High mortality probably also occurs during the aerial dispersal of San Jose scale crawlers, when only a small proportion would be expected to locate a suitable host plant. This will be countered to some degree by the wide host range of San Jose scale.
| Parasitoid species | Level of parasitism reported | |
| Aphytis diaspidis | Occurred in small numbers and increased late in the season on apples in Nelson, 1968-70. | |
| Aphytis mytilaspidis | Occurred in small numbers and increased late in the season on apples in Nelson, 1968-70. This species was found to be present even in low density populations of San Jose scale. | |
| Encarsia citrina | Dominant species on apples in Nelson 1968-70 where 20-88% parasitism occurred. It has also been reported to cause >90% mortality in some other populations, from Nelson northwards. | |
| Encarsia perniciosi | This species appeared to be increasing in importance on apples in the study by 1970. |
San Jose scale has established only in isolated pockets of the south of the South Island. For instance, it is confined to the Clyde/Earnscleugh area of Central Otago. This appears well within the limits of the climatic range of the species. Overseas research indicates that high winter mortality may occur in cool climate zones, particularly of those stages which are older than the first instar 'black cap' stage.