Sorghum Aphid

Common aphids in sorghum.

In warm weather, the sorghum aphid is pale yellow, with short, black cornicles (tail pipes). In colder weather, it can be various shades of brown or bluish gray. The only other aphid in sorghum with black cornicles is the corn leaf aphid, but in this species the head and legs are also dark. The only other yellow aphid in sorghum is the yellow sugarcane aphid, which has a more flattened body and is covered with fine bristles. The winged forms (alates) are darker, with a series of transverse lines across the back. The sorghum aphid feeds on the undersides of the leaves, forming very dense colonies and producing copious amounts of honeydew, which can give the upper leaf surfaces a slick, shiny appearance that can be visible from some distance.

The sorghum aphid cannot overwinter further north than the Oklahoma-Texas border, and survives the winter mostly on resprouting sorghum stubble in southern Texas and Louisiana. Johnsongrass is also an important wild host; although it is not as suitable a host plant as sorghum, it can be an important reservoir of aphids, supporting the development of the first generation in spring before sorghum becomes available. Kansas sorghum can become infested when sorghum aphids escape control in southern latitudes and produce large numbers of winged forms that are carried northward on southerly summer winds. Consequently, planting date has a significant effect on the susceptibility of sorghum to infestation in Kansas; the later the planting, the greater the risk of infestation by migrating sorghum aphids, and the greater the potential yield impact if colonies escape biological control on plants in earlier growth stages.

Sorghum aphids can infest the crop at any growth stage, although heavy populations do not usually develop until canopy closure. Similar to the corn leaf aphid, the sorghum aphid is able to feed in sorghum panicles well into the period of grain fill, until the black layer forms. In addition to reducing grain yield, it produces large volumes of honeydew that can impede harvest.

The best management practice for sorghum aphid in Kansas is to avoid late plantings. If patches of infested plants are discovered, check for the presence of lady beetles and the larvae of lacewings. If these are abundant, they are often able to eliminate the infestation within a matter of days. Be on the lookout for black, flattened aphids that are mummies of the aphid parasitoid Aphelinus varipes, or swollen, tan-colored mummies that indicate parasitism by Lysiphlebus testaceipes. These are evidence that many aphids are likely parasitized and will die without reproducing.

The economic threshold is considered to be 20% of plants infested with more than 50 aphids per leaf in vegetative stages, and 30% of plants in reproductive stages.

A detailed guide for scouting sorghum aphids can be found here: https://www.myfields.info/sites/default/files/page/ScoutCard%20KSU%20v05312017.pdf

It is important not to spray fields that are below the economic threshold as this will risk disrupting biological control that can otherwise be more effective than a pesticide application. Since sorghum aphids do not infest seedling plants at this latitude, there is no benefit to planting sorghum seed treated with systemic insecticides in Kansas, and the practice may actually diminish biological control of aphids and other sorghum pests, as plants without any insects lack food for natural enemies and will not be colonized by predators and parasitoids. If treatment is deemed necessary, the active ingredients sulfoxaflor and flupyradifurone will have reduced impact on natural enemies compared to other alternatives. Organophosphates and pyrethroids should not be used, as field tests suggest they harm natural enemies disproportionately, so that any aphids surviving the treatment have an even greater chance of escaping biological control.