Each year aphid infestations and BYDV infections start from aphids moving into wheat fields from other hosts. There are many other hosts for the aphid, nearly all grasses, and these hosts often serve as reservoirs for the virus. As the aphids arrive in the field small colonies will result. If the aphids carry the virus, small clusters of infected plants will result. You will not be able to see symptoms of infection at this stage, only the presence of aphids. If the fall migration is large lots of these little spots will occur.

A small fall aphid migration will result in relatively few aphid colonies and, therefore, fewer possible spots of BYDV infection.