Elevated CO2 Enhances the Effectiveness of Foliar Applications of Bt
Coviella, C.E. and Trumble, J.T. 2000. Effect of elevated atmospheric carbon dioxide on the use of foliar
application of Bacillus thuringiensis. BioControl 45: 325-336.
Toxins produced by Bacillus thuringiensis (Bt) supplied to crop plants via foliar application have been
used as a means of combating crop pests for more than fifty years. The effectiveness of this pest
management technique depends primarily upon the amount of Bt-produced toxins that are ingested by
susceptible insects. Hence, the authors wondered if atmospheric CO 2 enrichment might enhance the
effectiveness of the technique by inducing susceptible insects to eat more treated foliage.
The rational for this possibility derives from the fact that, if soil nitrogen levels are low, foliar nitrogen
concentrations are generally reduced from what they are at the current atmospheric CO 2 concentration;
and insects thus have to eat more foliage to get their normal requirement of nitrogen for proper growth
and development. However, in eating more foliage, the insects would also ingest more Bt-produced
toxins in the process, and they would thus be more severely impacted by the toxins.
What was done
To test this hypothesis, the authors grew cotton plants (Gossypium hirsutum L., Deltapine DP5415) in
twenty 3-liter pots in each of six Teflon-film chambers in a temperature-controlled greenhouse. Three of
these chambers were maintained at an atmospheric CO2 concentration of 370 ppm, and three were
maintained at 900 ppm CO2. In addition, half of the plants in each chamber received high levels of
nitrogen (N) fertilization, while half received low levels (130 vs. 30 mg N/kg soil/week).
After 45 days, leaves were removed from the plants and dipped in a solution containing Bacillus
thuringiensis var. aizawai, after which known amounts of treated leaf material were fed to Spodoptera
exigua (Hubner) larvae. A number of larval responses were then measured and analyzed.
What was learned
First, plants grown in the elevated CO2 chambers did indeed have significantly lower foliar nitrogen
concentrations than plants grown in the ambient CO2 chambers under the low N fertilization regime; but
this was not the case under the high N regime. Second, older larvae fed with foliage grown in elevated
CO2 with low N fertilization consumed significantly more plant material than insects fed with foliage
grown in ambient CO2; but, again, no differences were observed with high N fertilization. Third,
"consistent with the effect of higher Bt toxin intake due to enhanced consumption," the authors found
that "insects fed on low N plants had significantly higher mortality in elevated CO 2." And, again, no such
effect was evident in the high N treatment.
What it means
With respect to the important agricultural technique of pest management using toxins produced by
Bacillus thuringiensis supplied to crops via foliar applications, the authors state that "increasing
atmospheric CO2 is making the foliar applications more efficacious." Hence, science has documented yet
another way in which the ongoing rise in the air's CO2 concentration is helping the farmers of the world
produce the food upon which we all depend for our sustenance.