Recruitment of Natural Enemies of the Invasive Sugarcane Aphid Vary Spatially and Temporally in Sorghum Fields in the Southern Great Plains of the USA

Sorghum (Sorghum bicolor (L.) Moench) is an important summer grain crop in the U.S. Southern Great Plains because it is one of the few crops that consistently produces acceptable yields in the harsh summer weather that characterizes the region. Damaging infestations of the sugarcane aphid, Melanaphis sacchari (Zehntner), occur commonly in sorghum throughout Texas and Oklahoma, two of the leading sorghum producing states. Grain sorghum fields were sampled at two locations in Texas and three locations in Oklahoma. Fields were sampled approximately weekly by examining two leaves on each of 54 plants and counting all sugarcane aphids and aphid natural enemies on each leaf. Sampling began at an early growth stage and continued until the crop either matured or was treated with insecticide to suppress the sugarcane aphid infestation. For a total of 123 fields in 2017 and 2018, aphids, Aphelinus nigritus Howard mummies, adult coccinellids, larval coccinellids, larval lacewings, and larval dipterans were counted, and relative density estimates were determined. When natural enemy and sugarcane aphid count data were aggregated at the scale of geographic locations and multiple years, there was no evidence for a numerical response by natural enemies to sugarcane aphid density. When fields were compared within locations, a numerical response was consistently observed. A natural enemy importance index was developed that incorporated cumulative degree days of first occurrence of a natural enemy taxa in a field, the average density of the natural enemy in a field, and natural enemy voracity. Factorial analyses of variance indicated that cumulative degree days at first occurrence and average relative density differed significantly among natural enemy taxa and locations, as did natural enemy importance. Averaged across locations larval coccinellids had the largest importance index, I = 1.47, and A. nigritus had the smallest, I = 0.61. Among locations, the Texas Coastal Plains had the largest importance index, I = 1.27 whereas the Oklahoma Panhandle had the smallest, I = 1.02. Results suggest that differences occur in the biological control contributions of various natural enemies and in biological control efficacy among locations.

Automated summary

Sorghum is an important summer grain crop in the U.S. Southern Great Plains, but sorghum fields in Texas and Oklahoma are commonly infested by sugarcane aphids. Research indicates that there is no significant numerical response by natural enemies to sugarcane aphid density across different locations and years, but a numerical response is consistently observed when comparing fields within the same locations.