San Jose scale - Dispersal

Only the winged adult males and the young crawlers of San Jose scale are mobile. The threshold temperature for male flight is about 17^oC and they fly around sunset (or earlier on cloudy days) in wind speeds up to 1.8m/s. The immobile adult females produce a pheromone which attracts males for mating and this results in dispersal of males between San Jose scale populations. However, the primary means 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. Maximum release of crawlers of some scale insects is soon after dawn, and most settle on the same day, often during the night. 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 San Jose 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. Studies within an unsprayed apple orchard in Nelson showed that the settling of wind-blown crawlers of San Jose scale was usually on the shoots at the top of previously uninfested trees

The wide host range of San Jose 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 San Jose 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 San Jose scale on other host plants around orchards and on shelter can make a major contribution to scale cultural control.