Background
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 CO2 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 CO2 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 CO2."  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.