Woolly apple aphid - Dispersal

The dispersal of woolly apple aphid depends largely on the mobile first stage (instar) crawlers. A small proportion of adult females are winged in summer (from February onwards) but they do not disperse to elm, as occurs in North America, and the sexual forms, which they produce, are apparently sterile under New Zealand conditions. The movement of crawlers is strongly influenced by weather conditions, particularly temperature. Crawlers are active at temperatures above 10°C and movement has been reported to increase with rising temperatures from 13 to 26°C. Field studies have shown that many crawlers move up and down apple trees in spring, summer, and autumn, particularly during the warmer temperatures of the day. Very little movement occurs during the hours of darkness. It is the mobile crawlers which settle in the calyx and stem end of fruits and establish colonies there. The movements in spring have sometimes been interpreted to include upward movement from the roots to the tree, and in autumn to include downward movement from the tree to the roots. However, there are conflicting reports on this behaviour. Rainfall has been reported sometimes to reduce crawler movement. Colony size and crowding have been found to influence crawler dispersal, which increases with increasing density of the aphids in the colonies. Crawlers may wander for 2-3 days before settling to feed. This includes dispersal over the ground in search of new host plants and, on smooth surfaces, crawlers have been recorded walking 30 cm in 5 minutes. With this poor mobility, the dependence on crawlers for dispersal means that woolly apple aphid populations in New Zealand spread slowly between trees and orchards. Infestations are aggregated and often occur in small areas of the orchard. Aerial dispersal of crawlers has not been reported and the limited host range of woolly apple aphid restricts the sources of dispersing populations. It has been suggested that some woolly apple aphids are transported on the feet of birds.

A major source of dispersal for woolly apple aphid in the past has been on young trees and nursery stock. The careless distribution of infested trees rapidly spreads the aphid to new sites much more effectively than its crawlers.