Mussel scale - Life cycle

The mussel scale has one generation per year in most areas of New Zealand, with little overlap between the different stages. However, it may have a partial second generation in warmer North Island localities. The timing of events in the life cycle is well synchronised with the seasonal growth of the host plant and may be earlier in the warmer northern regions. Temperatures in spring and summer influence the timing of the different stages which may likewise be advanced or delayed by 1-2 weeks in some seasons.

Mussel scale spends the winter as cold-resistant eggs which are dormant (obligatory diapause). They are protected over the winter by being within the scale cover of the adult female scale which dies at the end of egg laying in summer/autumn. In Canterbury, most eggs hatch over 14 days within the period late September to early November. Large swarms of crawlers move out from under the female scale, many settling close by while other disperse more widely. They often settle under the empty scales of previous generations which persist for several years on the bark. The settled first instar crawlers moult to the second instar (stage) mainly in November and December, and adult females are produced by the next moult, beginning in early January. Egg laying begins in late January/February and continues into March before the females die in late autumn and early winter. As a result, mussel scale can be in the egg stage for about 9 months of the year. Populations of mussel scale throughout the world may have both males and females (sometimes called Lepidosaphes ulmi bisexualis) or be parthenogenetic (only female - sometimes called Lepidosaphes ulmi ulmi). Populations on apple trees tend to be parthenogenetic, with few or no males, as in New Zealand.

The rate of development of mussel scale in spring and summer depends on temperature. Further information is available on the life history of mussel scale.