Sugarcane aphid dynamics in Louisiana: identification of biochemical bases of resistance

Sugarcane in Louisiana is infested with two aphid species; the sugarcane aphid, Melanaphis sacchari (Zehntner), and the yellow sugarcane aphid, Sipha flava (Forbes). The sugarcane aphid is the main vector of sugarcane yellow leaf virus disease and is distributed throughout Louisiana’s sugarcane growing regions. The two aphids were studied on five sugarcane cultivars under field conditions for two years. We found that the sugarcane aphid is the dominant aphid species on sugarcane, and population differences occurred on different cultivars. Also, ratoon sugarcane had greater infestations than plant sugarcane. Cultivars L 97-128 and Ho 95-988 were the most susceptible cultivars and HoCP 91-555 and HoCP 96-540 were the most resistant cultivars season-long. The differences in nutritional value of the most susceptible (L 97-128) and most resistant (HoCP 91-555) cultivar were assessed through analyses of free amino acids in phloem sap and honeydew of sugarcane aphid. Absence of essential free amino acids in phloem sap or inability of the sugarcane aphid to synthesize certain free amino acids on the resistant cultivar likely contribute to the differences in host plant nutritional quality that can play a role in conferring resistance.