Sugarcane Aphid¹ Spatial Distribution in Grain Sorghum Fields

Sorghum, Sorghum bicolor (L.) Moench, is an important summer grain crop in the United States. In 2014, the U.S. produced 432 million bushels of sorghum valued at1.67 billion on more than 2.43 million ha. The sugarcane aphid, Melanaphis sacchari (Zehntner), was discovered in damaging numbers in grain sorghum in Texas and Louisiana in 2013. The sugarcane aphid can be very damaging to sorghum grown for grain and forage and is capable of explosive population growth. An objective sampling plan based on knowledge of spatial distribution of the aphid in sorghum would be useful for pest management decision making. We sampled 94 commercial grain sorghum fields in five southern states during 2 years to obtain data that were used to assess several aspects of sugarcane aphid distribution within and among sorghum plants in commercial grain sorghum fields. There were frequently significant differences in number of aphids among upper and lower leaves in fields in each state. However, there were no consistent patterns as to whether the upper or lower leaf had more sugarcane aphids. Autocorrelation coefficients were calculated for consecutive plants and for plants separated by one and two intervening plants. Numbers of sugarcane aphids on adjacent plants and plants separated by intervening plants were not autocorrelated, indicating that there was no, or only slight, correlation in numbers of sugarcane aphids among plants in close proximity to one another. Taylor's power law parameters for linear regression models relating the variance to the mean of numbers of sugarcane aphids on sorghum plants differed significantly for fields sampled in different states, but not for fields in vegetative versus reproductive growth stages, fields with markedly different planting dates (early or late), or fields sampled in different years.