Characterization of honeydew sugar profiles in sugarcane aphids on sorghum and Johnson grass

The sugarcane aphid, Melanaphis sacchari, (SCA) (Hemiptera: Aphididae) is an economically important
pest of grain sorghum, Sorghum bicolor, in the southern United States. The aphid reproduces
parthenogenetically, and under favorable conditions, populations can exceed a thousand aphids per
leaf. This is problematic for sorghum growers because SCAs can reduce yields dramatically. When
feeding, this aphid produces copious amounts of honeydew, which can also impede sorghum production
by clogging up farming equipment. Although SCA is a polyphagous pest, in the US it is mostly found on
grain sorghum and Johnson grass, Sorghum halepense, a perennial weed distributed throughout
sorghum growing regions. Interestingly, there appears to be a lower abundance of SCA natural enemies
on Johnson grass than on sorghum, even though the average number of SCAs on both host plants does
not significantly differ. This is important, because the uncultivated grass frequently occurs in proximity
to sorghum and is a host for SCA several weeks before sorghum is available. Thus, if few natural enemies
challenge SCA in Johnson grass, SCA populations can grow rapidly, spilling over into sorghum and
reaching levels above economic threshold. Considering that some predators and parasitoids consume
aphid honeydew as a non-prey carbohydrate source, this study aims to explore whether honeydew
sugar composition may explain differences in natural enemy abundance between Johnson grass and
sorghum. As such, this study characterized honeydew sugar composition between SCAs fed on grain
sorghum and Johnson grass.