Mussel scale - Dispersal

The winged males of mussel scale are very rare or absent from New Zealand populations and the immobile adult females produce female young (reproduce parthenogenetically). This means that only the young crawlers of mussel scale are mobile for the short period between egg hatch and settlement. They hatch from eggs under the mother scale cover and leave through a small space at the rear end. They crawl actively on twigs and branches and sometimes reach the leaves and fruits. The time spent crawling has been variously estimated as 15 minutes to 2 hours, and many have been observed to settle close to their mother scale. A large part of those which settle on the leaves die, whereas there is better survival on the twigs, bark and fruits. Crawlers also disperse aerially with the assistance of wind, although movement of crawlers of a related scale species was slowed by wind speeds of 4 m/s and ceased at 8 m/s. Wind dispersal is probably a high risk strategy resulting in many crawlers failing to locate a suitable host plant. Monitoring of crawlers of scale insects with sticky traps indicates that they may move several hundred metres downwind. There is also circumstantial evidence that crawlers (of various species) can be picked up in convection currents and carried many kilometres. The terminal velocity of crawlers in still air has been estimated at 0.12 m/s, indicating that upward currents could easily carry them long distances. Another method of dispersal reported for mussel scale is by hitching a ride on birds, several species of beetles, and ants.

The wide host range of mussel scale means that it may be present in the environment around orchards, and may be very close on shelter trees. Shelter trees and other host plants which are susceptible to mussel scale may be a source of scale control problems due to the dispersal of crawlers. Removal of infested plants around orchards can make a contribution to scale cultural control. However, mussel scale does not often reach the extremely high densities of oystershell scale and San Jose scale on shelter, probably due to the greater actions of mussel scale natural enemies and possibly due to competition from these more aggressive scale species.