Oystershell scale - Population dynamics

There are no published papers describing the population dynamics of mussel scale on apples in New Zealand. However, a detailed study of mussel scale populations on apple was carried out in Canada in the 1960s. This work showed that there was very high natural mortality, particularly from natural enemies. The parasitoid wasp, Aphytis mytilaspidis, and the predatory mite, Hemisarcoptes malus, were shown to be key factors regulating the population density of mussel scale at low levels. They particularly affected the survival of eggs, first stage nymphs, and egg-laying females. Total mortalities in (annual) generations of mussel scale ranged from 96.44% to 99.98%, and a total mortality of 98.44% allowed the population to maintain itself at the same level from one year to the next. Average predation of eggs by H. malus in different generations ranged from 71-85%, of 1st stage nymphs from 21-63%, of 2nd stage nymphs from 34-67%, and of adults from 24-63%. Parasitism of adult scales ranged up to 67% and egg production (fecundity) was reduced by 51-69% of the normal female, depending on the site and season.The two key natural enemies were also assisted by other predatory mites, parasitoid wasps, predatory thrips and ladybirds. Mortality of crawlers occurred through desiccation (during dispersal) and other stages died from overcrowding at high density and from frost. Reports suggest that mussel scale in Europe is also subject to high mortality from natural enemies which usually keep populations at low levels. This includes major contributions from the key species A. mytilaspidis and H. malus.

New Zealand has benefited from the accidental introduction of A. mytilaspidis but the apparent failure of H. malus to establish from introductions in the 1960s may account for the high densities of mussel scale sometimes encountered on apple trees. Reintroduction of this species or further natural enemies could be considered to prevent economic damage from this pest.