Yesterday I looked at some Johnsongrass in Millstead, Prattville, Autaugaville, and Marion Junction, AL. I found sugarcane aphids (Melanaphis sacchari) at one third of the sites surveyed. At those sites, no more than 30% of the Johnsongrass was infested.
Populations are just beginning to build, with aphid numbers on infested plants ranging from 1-20 aphids per plant. To add to the confusion of scouting, there are various aphids that can occur on Johnsongrass and sorghum. For example, yesterday I found yellow sugarcane aphid (Sipha flava) at two sites. Yellow sugarcane aphids are lemon yellow and seem thinner than sugarcane aphid when viewed without a hand lens.
Sugarcane aphids are usually cream to peach in color. In the photo above, courtesy of Dr. Charles Ray, the yellow sugarcane aphid is at the lower left. A hand lens or microscope clearly shows differences. Sugarcane aphid has black-tipped antennae, tailpipes (siphunculi) and feet (tarsi).
Yellow sugarcane aphid is spotted, hairy, and has no tailpipes. Only the tips of the feet are black. Other aphids that could be found on Johnsongrass and sorghum include corn leaf aphid, greenbug, English grain aphid, and hedgehog grain aphid.
Sorghum growers should be vigilant in scouting their fields for sugarcane aphids. A reasonable threshold is to treat with a registered insecticide if 20-30 percent of plants have 50 or more aphids per leaf (count one lower and one mid leaf on at least 10 plants). Sivanto Prime and Transform WG can be used on forage and grain sorghum in Alabama in 2017.