The greenbug, Schizaphis graminum (Rondani), can kill seedlings and reduce the yield of older sorghum, Sorghum bicolor (L.) Moench, and small grains. The objective of this study was to determine effects of different amounts of soil water (−33, −50, −100, and −300 kPa) and nitrogen (21, 50, 100, and 150 ppm) on fitness of biotype I greenbugs on sorghum. The pre-reproductive period of a greenbug ranged from 5.6–7.3 days but was not affected significantly by soil moisture or nitrogen. Lack of soil moisture, but not nitrogen, adversely affected greenbug fecundity and longevity. Significantly most nymphs (44.5) were produced per greenbug and greenbugs lived longest (25.9 and 31.2 days) on sorghum in soil with adequate field-capacity moisture (−33 kPa).
Plants were tallest and produced twice as much dry weight in soil with −33 kPa (67.6 cm tall and 3.7 g per plant) compared with −300 kPa of water potential (56.6 cm and 1.9 g, respectively). The amount of nitrogen in plants in soil with the greatest (−33 kPa) and least (−300 kPa) water potentials was 0.93–0.94%, and was least (0.81%) at the intermediate −100 kPa of water. Plants were tallest (65.8 cm) and produced most dry weight (3.5 g per plant) in soil with 150 ppm of nitrogen, and were shortest (55.2 cm) and produced least dry weight (2.1 g) in soil with only 21 ppm of nitrogen. Plants in soil with the greatest and least amounts of nitrogen had 0.96 and 0.81% nitrogen, respectively. The percentage of nitrogen was greatest (1.08%) in plants in soil with a combination of least moisture (−300 kPa) and most nitrogen (150 ppm).