Effect of elevated atmospheric carbon dioxide on the use of foliar application of Bacillus thuringiensis

Abstract

Toxins from Bacillus thuringiensis have been used as pest management tools for more than 50 years. The effect of these toxins depends on the quantity of Bacillus thuringiensis (Bt) toxins ingested by susceptible insects. Food ingestion is affected by CO2 concentration; plants grown in elevated CO2 often have increased carbon/nitrogen ratios (C/N), resulting in greater leaf area consumption. Therefore, we hypothesized that elevated CO2 would improve the efficacy of foliar applications of B. thuringiensis. Cotton plants were grown at either ambient (360–380 l/l) or elevated CO2 (900 l/l). Groups of plants in both CO2 treatments were exposed to low (30 mg/kg soil/week) or high (130 mg/kg soil/week) nitrogen (N) fertilization levels in a split plot design. The resulting plants were assessed for N and carbon (C) contents. Leaf disks from the same plants were dipped in a Bt solution and then fed to Spodoptera exigua (Hübner), an insect species of considerable economic importance. Elevated CO2 significantly reduced total N, and increased the C/N. Nitrogen fertilization significantly affected consumption by early stadia larvae, larval weight gain, and relative growth rate (RGR). Interactions between CO2 concentration and N fertilization level significantly impacted late stadia larval food consumption, and through differential Bt toxin intake, affected duration of larval stage and mortality to the adult stage.We conclude that the elevated atmospheric CO2 concentrations expected in the next century will interact with commercial fertilization practices to enhance the efficacy of B. thuringiensis formulations applied topically to crops. The implications for improved control are discussed.