San Jose scale - Population dynamics

There are no published papers describing the population dynamics of San Jose scale on apples in New Zealand. An unpublished study from 1969 to 1970, at the Appleby Research Orchard in Nelson, investigated a population of San Jose scale on an individual unsprayed apple tree, including its distribution within the tree, density of stages, and causes of mortality. Although high levels of parasitism and some fungal disease were found in this research, it was insufficient to prevent the scale population from rising to cause severe damage to the tree and infesting the fruit. These results supported more extensive research at the Research Orchard which recorded a steady increase in San Jose scale populations on large blocks of apple trees from 1963 to 1972 in the absence of any spraying with oil or other insecticides. These data, and the high densities of San Jose scale reported on neglected apple trees, indicate that further natural enemies of San Jose scale are needed in New Zealand to prevent economic damage from this pest. Further information is available from overseas studies.

Female San Jose scale have high fecundity and the crawlers produced are primarily responsible for dispersal of San Jose scale populations. Huge numbers of crawlers are produced in dense populations and it is probable that those dispersing on wind current suffer high mortality, with only a small proportion reaching a suitable host plant, particularly when dispersing beyond orchards. This will be countered to some degree by the wide host range of San Jose scale.