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

Abstract

Toxins from Bacillus thuringiensis have beenused as pest management tools for more than 50 years. The effect of these toxins depends on the quantityof Bacillus thuringiensis (Bt) toxins ingestedby susceptible insects. Food ingestion is affected byCO2 concentration; plants grown in elevatedCO2 often have increased carbon/nitrogen ratios(C/N), resulting in greater leaf area consumption. Therefore, we hypothesized that elevated CO2would improve the efficacy of foliar applications ofB. thuringiensis. Cotton plants were grown ateither ambient (360–380 μl/l) or elevated CO2(900 μl/l). Groups of plants in both CO2treatments were exposed to low (30 mg/kg soil/week) orhigh (130 mg/kg soil/week) nitrogen (N) fertilizationlevels in a split plot design. The resulting plantswere assessed for N and carbon (C) contents. Leafdisks from the same plants were dipped in a Btsolution and then fed to Spodoptera exigua(Hubner), an insect species of considerableeconomic importance. Elevated CO2 significantlyreduced total N, and increased the C/N. Nitrogenfertilization significantly affected consumption byearly stadia larvae, larval weight gain, and relativegrowth rate (RGR). Interactions between CO2concentration and N fertilization level significantlyimpacted late stadia larval food consumption, andthrough differential Bt toxin intake, affectedduration of larval stage and mortality to the adultstage. We conclude that the elevated atmosphericCO2 concentrations expected in the next centurywill interact with commercial fertilization practicesto enhance the efficacy of B. thuringiensisformulations applied topically to crops. Theimplications for improved control are discussed.