Greenbug usually is considered a key insect pest of
sorghum. The aphid sucks juices from and injects toxin into plants. Small grains,
primarily wheat, are the winter host. Where the growing season of wheat does not overlap
that of sorghum, grasses such as johnsongrass, are interim hosts.Greenbugs feed in
colonies on the underside of leaves and produce much honeydew. The greenbug may be a pest
during the seedling stage of growth but often does not reach damaging numbers until the
sorghum panicle develops. Infestations may be detected by the appearance on leaves of
reddish spots caused by the toxin greenbugs inject. Reddened areas enlarge as the number
of and damage by greenbugs increase. Damaged leaves begin to die, turning yellow and then
brown from the outer edges. Damage to seedlings may result in stand loss. Larger sorghum
plants tolerate more greenbugs than do seedlings. Yield reductions during boot, flowering,
and kernel-development stages depend on greenbug abundance, length of time greenbugs have
infested plants, and plant health. Many greenbugs on booting and older plants reduce yield
because of fewer and smaller kernels that develop and weakened plants that may lodge
later. Greenbugs also transmit maize dwarf mosaic virus and may predispose sorghum to
charcoal rot.
Figure 5 illustrates the economic impact of greenbugs at different abundance levels and
plant growth stages. Yield of sorghum is affected economically when there are 500 to 1 000
greenbugs per plant. However, the number of greenbugs required to cause economic damage
depends greatly on the size and condition of sorghum plants.
