Oystershell scale - Population dynamics

There are no published papers describing the population dynamics of oystershell scale on apples in New Zealand. A study from July to December 1960 in a Canterbury apple orchard reported high levels of of mortality due to parasitism, mainly by the aphelinid wasp Encarsia citrina. Predation was also reported as common by a predatory camerobiid mite, Neophyllobius sp. Despite these reported major mortalities, oystershell scale reaches high population densities on unsprayed apple trees in New Zealand, often exceeding 100 scale insects/square cm and causing severe damage. Limited studies in Otago have discovered very high densities of oystershell scale on a range of unsprayed host plants and have not found the high levels of parasitism on apples reported from Canterbury. This contrasts with reports from Europe which say that oystershell scale (=European fruit scale) "is severely parasitised and it can rarely multiply excessively". This research, when taken with the high densities of oystershell scale reported on neglected apple trees here, indicate that further natural enemies are needed in New Zealand, and probably available overseas, to prevent economic damage from this pest.

Female oystershell scale have high fecundity and the crawlers produced are primarily responsible for dispersal of oystershell 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.