They spread either through some mechanical agency or directly by crawling. Birds and insects can also transport aphids. Any unsettling conditions will produce crawlers, though they are generally more abundant in spring and fall. Crawlers cannot work into and through the soil. In orchards, the swaying of trees in the wind, and organic matter, clods, stones and other objects may provide pathways to the roots.
Crawlers begin to infest the roots early in the season.
The downward movement can begin at any time when the crawlers are numerous, especially in early summer and fall. Infestations by aerial colonies are not true indications of root infestations over a season without the roots becoming infested. If numbers warrant treatment, spray during the early season before migration to the roots occurs. Control Some pesticides, such as pyrethroids and carbamates, encourage outbreaks. These should be used sparingly or be combined with a n aphicide.