What was done
The authors grew transgenic Bt cotton cultivar GK-121 [expressing Cry1A(c) genes from Bacillus thuringiensis Berliner var. kurstaki] as well as the non-transgenic cotton cultivar Simian-3 from the same recurrent parent in plastic pots filled with 8:3:1 (by volume) loam:cow-dung:earthworm frass, which they placed within open-top chambers in Sanhe County, Hebei Province, China that were continuously maintained at either ambient (375 ppm) or twice-ambient (750 ppm) atmospheric CO2 concentrations from the time of planting on 10 May 2004 until the plants were harvested in October, after which egg masses of the cotton bollworm - Helicoverpa armigera Hubner (Lepidoptera: Noctuidae) - were reared in a growth chamber under ambient-CO2 conditions, while three successive generations of them were fed either transgenic or non-transgenic cotton bolls from plants grown in either ambient or twice-ambient atmospheric CO2 concentrations, during which time a number of physiological characteristics of the cotton bollworms were periodically assessed.

What was learned
"Overall," in the words of Chen et al., "both elevated CO2 and transgenic Bt cotton increased larval lifespan," but they decreased "pupal weight, survival rate, fecundity, frass output, relative and mean relative growth rates, and the efficiency of conversion of ingested and digested food." As a result, they were also able to report that "transgenic Bt cotton significantly decreased the population-trend index compared to non-transgenic cotton for the three successive bollworm generations, especially at elevated CO2 [our italics]."

What it means
Based on their observations, the four researchers conclude that the negative effects of elevated CO2 on cotton bollworm physiology and population dynamics "may intensify through successive generations," in agreement with the findings of Brooks and Whittaker (1998, 1999) and Wu et al. (2006). Hence, they additionally conclude that "both elevated CO2 and transgenic Bt cotton are adverse environmental factors for cotton bollworm long-term population growth," and that "the combination of the two factors may intensify their adverse impact on the population performance of H. armigera," which should be especially good news for cotton growers.

References
Brooks, G.L. and Whittaker, J.B. 1998. Responses of multiple generations of Gastrophysa viridula, feeding on Rumex obtusifolius, to elevated CO2. Global Change Biology 4: 63-75.

Brooks, G.L. and Whittaker, J.B. 1999. Responses of three generations of a xylem-feeding insect, Neophilaenus lineatus (Homoptera), to elevated CO2. Global Change Biology 5: 395-401.

Wu, G., Chen, J.F. and Ge, F. 2006. Response of multiple generations of cotton bollworm Helicoverpa armigera Hubner, feeding on spring wheat, to elevated CO2. Journal of Applied Entomology 130: 2-9.