Soil moisture and larval western corn rootworm injury: Influence on gas exchange parameters in corn

The western corn rootworm (Diabrotica virgifera virgifera LeConte) is an economically important pest of corn (Zea mays L.) in North America, but little is known about the physiological responses of corn to larval corn rootworm damage. This study was conducted to (i) characterize how larval corn rootworm feeding affects corn photosynthetic rate (P-s), stomatal conductance (g(c)), and intercellular CO2 concentration (C-i) and (ii) determine the interactive effects of larval injury and soil moisture on P-s, g(c), and C-i. To address Objective 1, a greenhouse and a field experiment in randomized complete block design compared rootworm-infested vs. noninfested Pioneer hybrid 3377 corn. Larval feeding had significant and transient effects on leaf P-s and g(c) (but not C-i) in both experiments. When the rootworm population was predominately second instar, leaf P-s and g(c) in infested corn were significantly less (P < 0.05) than in noninfested corn; at the end of the larval period, P-s and g(c) were not significantly different among rootworm treatments. Two greenhouse experiments addressed Objective 2. A factorial design with high and low moisture levels and rootworm-infested and noninfested plants was used. In both experiments, soil moisture level interacted with larval injury to significantly (P < 0.05) affect leaf P-s and g(c) (but not C-i). Within 24 h after the two moisture regimes were established, P-s of the noninfested plants dramatically increased, while rates of rootworm-infested plants did not change in plants that received supplemental water. Over 5 d, P-s rose in rootworm-infested plants until rates equilibrated among rootworm treatments that were given supplemental water. These results suggest that irrigation of infested corn may not immediately reduce the stress caused by larval injury. A combination of factors (e.g., water uptake reduction, hormone synthesis in roots) may be involved in the injury-plant response process.