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

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 l/l) or elevated CO₂(900 l/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(Hübner), 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.