Mussel scale - Life history

Mussel scale populations live primarily on and under the bark of their host trees and it is difficult for insecticide sprays to reach these concealed insects. In addition, they spend 3/4 of their lives as eggs protected under the old scale of the mother. On twigs, they may often be found in winter tucked amongst the leaf scars below the buds, and they may form dense encrustations on branches. In other parts of the world, mussel scale occurs as both parthenogenetic (all female) and bisexual populations which differ in their host plants and egg production. Males are very rare or absent in New Zealand populations. The adult female mussel scale produces female eggs (without mating) over the summer, which accumulate under the female scale cover and stay there through the winter in an obligatory dormancy (diapause). The female dies at the end of egg laying. After the long incubation (8-9 months), the eggs hatch in spring and early summer into mobile crawlers. The eggs at the posterior end of the scale hatch first, and the crawlers disperse and settle on the bark and fruits. Many settle close to (or even under) the dead mother scale. Other crawlers are carried by the wind to other host plants. After settling, the young scale insects insert their feeding stylets into the phloem of the plant. Mussel scales also contain microorganisms which assist with their nutrition (endosymbionts). The first instar settled (immobile) crawlers secrete paired wax filaments that form a cap over their bodies which has a woolly appearance. The cap hardens with further wax secretions to form a scale. They then moult to the second instar (stage) and become increasingly elongate. Females undergo one further moult before becoming adult, but the rare males pass through prepupal and pupal stages before emerging as winged adults. As in northern Europe, the life cycle in southern New Zealand has one generation per year. A partial second generation may occurs in warmer northern areas. The number of generations per year in Europe is affected by the host plant as well as temperatures.

The threshold minimum and the maximum temperatures for mussel scale development in New Zealand are not known. Overseas studies indicate that a temperature sum of >130.8^oC in spring is required for egg hatch, and that a period of 8 weeks at 13^oC is sufficient to break the dormancy (diapause) of overwintering eggs and initiate development. The moult from 1st to 2nd stage nymph occurs about a month after hatching, and the second stage lasts 3-4 weeks. Adult females lay eggs for about 4-6 weeks.