While the sugarcane aphid has complicated the production of grain sorghum (milo) in Kentucky since its detection in late August, it has destroyed sweet sorghum in many Kentucky counties. Since the first detection of this new invasive aphid on grain sorghum in August in western Kentucky, there have been reports of sweet sorghum lost to this pest in five counties. It has been detected as far east as Casey and Garrard Counties. Growers are reporting that large aphid populations have resulted in lower sugar levels in the juice, less juice, killed stalks, and off-flavors in the syrup.
In early September the Environmental Protection Agency (EPA) granted an emergency exemption for use of Transform to control sugarcane aphid on sorghum used for grain and fodder; however this exemption for Transform does not permit use on sweet sorghum. Sivanto is the other insecticide that is labeled and effective against sugarcane aphid on sorghum, but as with Transform, it cannot be used on sweet sorghum. Last month the EPA lost a court decision regarding the labeling of sulfoxaflor, the active ingredient in Transform and Closer. Soon we will learn of impact of this decision on the use of sulfoxaflor. Lorsban, dimethoate, and malathion are labeled for controlling aphids on sorghum, but they are not as effective as Transform or Sivanto. Currently we have no conventional insecticides that are labeled for use on sweet sorghum other than Intrepid, which is only effective against caterpillars. The only organic insecticides labeled for control of aphids on sweet sorghum are the neem-seed based azadirachtin insecticides.
The reason for the limited number of insecticides most likely has to do with how sweet sorghum is processed. It is crushed for the juice, and the juice is boiled down for syrup (Figure 2) with 8 gallons of juice yielding 1 gallon of syrup.
Companies with interest in labeling pesticides for sweet sorghum would need to conduct the residue trials on the syrup as part of the registration process. The IR-4 Minor Use Program will begin the process this coming summer to conduct the needed residue studies to expand the Sivanto label to include sweet sorghum. This process typically takes 3 or more years to result in labeling expansion.
By Ric Bessin, Extension Entomologist