Background
In January of 1996, a 0.8-ha area of scrub-oak forest at NASA's Kennedy Space Center in Florida, USA, was burned; and during the following spring, 16 open-top chambers were constructed upon the site, eight of which were thereafter continuously maintained at the air's ambient CO2 concentration (which ranged from c. 350 ppm in 1996 to c. 380 ppm in 2007), while the other eight chambers were maintained at ambient + 350 ppm over the intervening decade.

What was done
Quoting the scientists who conducted the work within the 16 enclosures, "during August 2001 and 2002, we counted the numbers of leaf miners, leaf tiers, chewed leaves, eyespot galls, leaf galls and leaves with pathogens per 200 haphazardly selected leaves on each oak species [myrtle oak (Quercus myrtifolia Willd), sand-live oak (Quercus geminata Small), and Chapman oak (Q. chapmanii Sargenti)] in each chamber and on the legume Galactia elliottii," which they also did at various times thereafter.

What was learned
Stiling et al. report that across the years "elevated CO2 reduced the densities of all herbivore-damaged leaves, which included damage produced by leaf miners, leaf tiers, leaf chewers and leaf gallers, on all host plant species, including the nitrogen-fixing legume, Galactia," while additionally citing the paper of Stiling et al. (2009) in this regard. And they also write that their results "are similar to those of other studies, most of which have also found reductions in insect herbivory under elevated CO2 (reviewed in Lincoln et al., 1993; Watt et al., 1995; Bezemer and Jones, 1998; Hunter, 2001; Whittaker, 2001; Stiling and Cornelissen, 2007; Lindroth, 2010; Robinson et al., 2012)."

What it means
In the eternal war between earth's plants and the herbivorous insects that feed upon them, the ongoing increase in the atmosphere's CO2 concentration is proving to be a powerful secret weapon, which is helping the planet's vegetation to sustain the impressive worldwide greening of the earth that continues apace, growing ever stronger year by year.

References
Bezemer, T.M. and Jones, T.H. 1998. Plant-insect herbivore interactions in elevated atmospheric CO2, quantitative analyses and guild effects. Oikos 82: 212-222.

Hunter, M.D. 2001. Effects of elevated atmospheric carbon dioxide on insect-plant interactions. Agricultural and Forest Entomology 3: 153-159.

Lincoln, P.E., Fajer, E.D. and Johnson, R.H. 1993. Plant-insect herbivore interactions in elevated CO2. Trends in Ecology and Evolution 8: 64-68.

Lindroth, R. 2010. Impacts of elevated atmospheric CO2 and O3 in forests: phytochemistry, trophic interactions and ecosystem dynamics. Journal of Chemical Ecology 36: 2-21.

Robinson, E.A., Ryan, G.D. and Newman, J.A. 2012. A meta-analytical review of the effects of elevated CO2 on plant-arthropod interactions highlights the importance of interacting environmental and biological variables. New Phytologist 194: 321-336.

Stiling, P. and Cornelissen, T. 2007. How does elevated carbon dioxide (CO2) affect plant-herbivore interactions? A field experiment and meta-analysis of CO2-mediated changes on plant chemistry and herbivore performance. Global Change Biology 13: 1823-1842.

Stiling, P., Moon, D.C., Rossi, A.M., Hungate, B.A. and Drake, B. 2009. Seeing the forest for the trees, long term exposure to elevated CO2 increases some herbivore densities. Global Change Biology 15: 1895-1902.

Watt, A.D., Whittaker, J.B., Docherty, M., Brooks, G., Lindsay, E. and Salt, D.T. 1995. The impact of elevated atmospheric CO2 on insect herbivores. In: Harrington, R. and Stork, N.E. (Eds.). Insects in a Changing Environment: Symposium of the Royal Entomological Society. Academic Press, London, United Kingdom, pp. 198-217.

Whittaker, J.B. 2001. Insects and plants in a changing atmosphere. Journal of Ecology 89: 507-518.