Oystershell scale - Dispersal

Only the winged adult males and the young crawlers of oystershell scale are mobile. The immobile adult females produce a pheromone which attracts males for mating and this results in dispersal of males between oystershell scale populations. However, the main method of population dispersal is by movement of crawlers, which walk over the surface of the host tree and disperse aerially with the assistance of wind. Movement of crawlers of one scale species was slowed by wind speeds of 4 m/s and ceased at 8 m/s. Most settle within a few dozen centimeters of their release site, but theoretical calculations indicate that they could walk for 150 m. Survival is greater at high humidity, and shorter at higher temperatures. The high fecundity of oystershell scale results in very large numbers of crawlers dispersing by wind each summer, but this high risk strategy probably results 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 from their host plants. However, there is circumstantial evidence that crawlers 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. Monitoring of oystershell scale crawlers with sticky traps in Otago and Canterbury has shown a well synchronised start to the dispersal and a rapid rise to peak numbers in late December or mid January depending on the location and host plant. Crawler aerial dispersal then dropped to much lower levels and trickled on through January to April (see life cycle).

The wide host range of oystershell 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 oystershell scale are often a major source of scale control problems due to the dispersal of crawlers into the orchard during the summer. Reduction and removal of the reservoirs of oystershell scale on other host plants around orchards and on shelter can make a major contribution to scale cultural control.