What was done
The authors grew three genotypes of mouse-ear cress (Arabidopsis thaliana) from seed in pots within controlled-environment chambers maintained at either ambient CO2 (360 ppm) or elevated CO2 (720 ppm).  On each of half of the plants (the herbivory treatment) in each of these CO2 treatments, they placed two second-instar larvae of the diamondback moth (Plutella xylostella) at bolting time and removed them at pupation, which resulted in an average of 20% of each plant's total leaf area in the herbivory treatment being removed.  Then, each pupa was placed in a gelatin capsule until adult emergence and ultimate death, after which insect gender was determined and the pupa's weight recorded.  At the conclusion of this herbivory trial, leaves of the control and larvae-infested plants were analyzed for concentrations of individual glucosinolates - a group of plant-derived chemicals that can act as herbivore deterrents (Maruicio and Rausher, 1997) - while total glucosinolate production was determined by summation of the individual glucosinolate assays.  Last of all, influences of elevated CO2 on moth performance and its association with plant defense-related traits were evaluated.

What was learned
Overall, it was determined that herbivory by larvae of the diamondback moth did not induce any increase in the production of glucosinolates in the mouse-ear cress in the ambient CO2 treatment.  However, Bidart-Bouzat et al. report that "herbivory-induced increases in glucosinolate contents, ranging from 28% to 62% above basal levels, were found under elevated CO2 in two out of the three genotypes studied."  In addition, they determined that "elevated CO2 decreased the overall performance of diamondback moths."

What it means
Because "induced defenses can increase plant fitness by reducing subsequent herbivore attacks (Agrawal, 1999; Kessler and Baldwin, 2004)," the authors suggest that "the pronounced increase in glucosinolate levels under CO2 enrichment may pose a threat not only for insect generalists that are likely to be more influenced by rapid changes in the concentration of these chemicals, but also for other insect specialists more susceptible than diamondback moths to high glucosinolate levels (Stowe, 1998; Kliebenstein et al., 2002)."  Hence, it is tempting to speculate that the ongoing rise in the air's CO2 content will enable earth's vegetation to better withstand the ravages of marauding herbivores in the years and decades ahead.

References
Agrawal, A.A.  1999.  Induced-responses to herbivory in wild radish: effects on several herbivores and plant fitness.  Ecology 80: 1713-1723.

Kessler, A. and Baldwin, I.T.  2004.  Herbivore-induced plant vaccination. Part I. The orchestration of plant defenses in nature and their fitness consequences in the wild tobacco, Nicotiana attenuata.  Plant Journal 38: 639-649.

Kliebenstein, D., Pedersen, D., Barker, B. and Mitchell-Olds, T.  2002.  Comparative analysis of quantitative trait loci controlling glucosinolates, myrosinase and insect resistance in Arabidopsis thaliana.  Genetics 161: 325-332.

Mauricio, R. and Rausher, M.D.  1997.  Experimental manipulation of putative selective agents provides evidence for the role of natural enemies in the evolution of plant defense.  Evolution 51: 1435-1444.

Stowe, K.A.  1998.  Realized defense of artificially selected lines of Brassica rapa: effects of quantitative genetic variation in foliar glucosinolate concentration.  Environmental Entomology 27: 1166-1174.