Corn Leaf Aphid, Rhopalosiphum maidis.
Note dark legs and cornicles, relatively short antennae
Corn leaf aphids can be quite pale in color when they develop in hot weather.
Sorghum whorl infested with corn leaf aphids.
The corn leaf aphid, Rhopalosiphum maidis, frequently infests sorghum at the whorl stage. This aphid ranges in color from dark bluish-green to pale yellow, depending on temperature, darker forms being associated with cooler conditions. Key identifying features are the dark legs and cornicles, relatively short antennae, and feeding on the upper leaf surfaces exclusively, preferring the upper parts of the plant. Greenbugs, in contrast, have pale legs and cornicles, antennae almost as long as the body, and feed exclusively on the undersides of sorghum leaves, preferring leaves on the lower parts of the plant. Initial winged migrants arrive on the wind from southerly latitudes (or earlier planted corn fields) and initiate colonies deep in the whorl where they are detectable only by pulling out the central leaf and unrolling it. Observing the activities of lady beetles is the easiest way to locate infested plants at this stage. Although large colonies may remove substantial volumes of sap, they do not cause direct plant injury. Their feeding occurs largely during the period of strong vegetative growth when plants are most able to compensate for these losses. Usually, corn leaf aphid colonies disappear before the panicle emerges fully, the majority of nymphs maturing into winged adults and dispersing in search of other plants in suitable growth stages.
Studies performed over many years in the Texas panhandle have shown that whenever corn leaf aphid populations peak at or before boot stage in sorghum, greenbug infestations do not become economic later on. This is because a diverse guild of aphid natural enemies is attracted by them, most of which are able to complete a generation and thus augment their numbers prior to subsequent colonization of the crop by greenbugs. Therefore, it is important for growers to distinguish between these two aphid species as unwarranted treatments of corn leaf aphids not only represent a waste of money, but may actually diminish subsequent natural control of the more damaging greenbug by eliminating beneficial species. Because corn leaf aphids do little damage and attract many beneficial species, they are rarely a cause for concern unless large numbers persist into heading.
Page last updated on 10/31/2013 by J.P. Michaud.