Stiling et al. (1999) enclosed portions of a Florida scrub-oak community in open-top chambers and maintained them at atmospheric CO2 concentrations of 350 and 700 ppm for approximately one year, while they studied the effects of this treatment on destructive leaf miners.  Among their many findings, the researchers noted that the individual areas consumed by leaf miners munching on leaves in the CO2-enriched chambers were larger than those created by leaf miners dining on leaves in the ambient-air chambers.  As a result, there was a four-fold increase in parasitization by various wasps that could more readily detect the more-exposed leaf miners on the CO2-enriched foliage.  Consequently, leaf miners in the elevated CO2 chambers suffered significantly greater mortality than those in the control chambers.

In a subsequent and much expanded study of the same ecosystem, Stiling et al. (2002) investigated several characteristics of a number of insect herbivores found on the five species of plants that accounted for over 98% of the total plant biomass within the chambers.  As they describe their results, the "relative levels of damage by the two most common herbivore guilds, leaf-mining moths and leaf-chewers (primarily larval lepidopterans and grasshoppers), were significantly lower in elevated CO2 than in ambient CO2, for all five plant species."  In addition, they report that "more host-plant induced mortality was found for all miners on all plants in elevated CO2 than in ambient CO2."  These effects were so powerful, in fact, that in addition to the relative densities of insect herbivores being reduced in the CO2-enriched chambers, and "even though there were more leaves of most plant species in the elevated CO2 chambers," the total densities of leaf miners in the high-CO2 chambers were also lower for all plant species.

In another study of oak trees, Dury et al. (1998) grew four-year-old Quercus robur saplings in pots placed within greenhouses maintained at ambient and twice-ambient atmospheric CO2 concentrations in combination with ambient and elevated (ambient plus 3°C) air temperatures.  Although no herbivores were included in the experiment, the authors determined that elevated CO2 would "not [be] expected to affect nutritional quality of foliage for spring-feeding larvae."  The increase in temperature, however, increased leaf toughness and led them to conclude that insects feeding on such foliage would experience prolonged larval development and reduced growth.

In another study that did not involve herbivores, Gleadow et al. (1998) grew eucalyptus seedlings in glasshouses maintained at 400 and 800 ppm CO2 for a period of six months, observing biomass increases of 98% and 134% in high and low nitrogen treatments, respectively.  They also studied a sugar-based compound called prunasin, which produces cyanide in response to tissue damage caused by foraging herbivores.  Although elevated CO2 caused no significant change in leaf prunasin content, it was determined that the proportion of nitrogen allocated to prunasin increased by approximately 20% in the CO2-enriched saplings, suggestive of a potential for increased prunasin production had the saplings been under attack by herbivores.

In a study of simulated herbivory, Kruger et al. (1998) grew seedlings of one-year-old maple (Acer saccharum) and two-year-old aspen (Populus tremuloides) in glasshouses with atmospheric CO2 concentrations of 356 and 645 ppm for 70 days.  At the 49-day point of the experiment, half of the leaf area on half of the trees in each treatment was removed.  This defoliation caused the final dry weights of both species to decline in the ambient-air glasshouse.  In the CO2-enriched glasshouse, on the other hand, the defoliated maple trees ended up weighing just as much as the non-defoliated maple trees; while the defoliated aspen trees ended up weighing a little less, but not significantly less, than their non-defoliated counterparts.  Hence, atmospheric CO2 enrichment helped both species to better recover from the debilitating effect of leaf removal, which suggests that these trees may be better able to deal with the physical damage that might be inflicted upon them by herbivores in a future world of higher atmospheric CO2 concentration.

In another study of mechanical defoliation, Lovelock et al. (1999) grew seedlings of the tropical tree Copaifera aromatica for 50 days in pots placed within open-top chambers maintained at atmospheric CO2 concentrations of 390 and 860 ppm.  At the 14-day point of the experiment, half of the seedlings in each treatment had about 40% of their total leaf area removed.  In this case, none of the defoliated trees of either CO2 treatment fully recovered from this manipulation; but at the end of the experiment, the total plant biomass of the defoliated trees in the CO2-enriched treatment was 15% greater than that of the defoliated trees in the ambient-CO2 treatment, again attesting to the benefits of atmospheric CO2 enrichment in helping trees to deal with herbivory.

Docherty et al. (1997) grew beech and sycamore saplings in glasshouses maintained at atmospheric CO2 concentrations of 350 and 600 ppm, while groups of three sap-feeding aphid species and two sap-feeding leafhopper species were allowed to feed on them.  Overall, elevated CO2 had few significant effects on the performance of the insects, although there was a non-significant tendency for elevated CO2 to reduce the individual weights and populations sizes of the aphids.

Finally, Hattenschwiler and Schafellner (1999) grew seven-year-old spruce (Picea abies) trees at atmospheric CO2 concentrations of 280, 420 and 560 ppm and various nitrogen deposition treatments for three years, allowing nun moth larvae to feed on current-year needles for a period of 12 days.  Larvae placed upon the CO2-enriched foliage consumed less needle biomass than larvae placed upon the ambiently-grown foliage, regardless of nitrogen treatment.  In fact, this effect was so pronounced that the larvae feeding on needles produced by the CO2-enriched trees attained an average final biomass that was only two-thirds of that attained by the larvae that fed on needles produced at 280 ppm CO2.  Since the nun moth is a deadly defoliator that resides in most parts of Europe and East Asia between 40 and 60°N latitude and is commonly regarded as the coniferous counterpart of its close relative the gypsy moth, which feeds primarily on deciduous trees, the results of this study thus suggest that the ongoing rise in the air's CO2 content will likely lead to significant reductions in damage to spruce and other coniferous trees by this voracious insect pest in the years and decades ahead.

In light of these several observations, the balance of evidence seems to suggest that earth's non-woody plants will be better able to deal with the challenges provided by herbivorus insects as the air's CO2 content continues to rise.

References
Docherty, M., Wade, F.A., Hurst, D.K., Whittaker, J.B. and Lea, P.J.  1997.  Responses of tree sap-feeding herbivores to elevated CO2.  Global Change Biology 3: 51-59.

Dury, S.J., Good, J.E.G., Perrins, C.M., Buse, A. and Kaye, T.  1998.  The effects of increasing CO2 and temperature on oak leaf palatability and the implications for herbivorous insects.  Global Change Biology 4: 55-61.

Gleadow, R.M., Foley, W.J. and Woodrow, I.E.  1998.  Enhanced CO2 alters the relationship between photosynthesis and defense in cyanogenic Eucalyptus cladocalyx F. Muell.  Plant, Cell and Environment 21: 12-22.

Hattenschwiler, S. and Schafellner, C.  1999.  Opposing effects of elevated CO2 and N deposition on Lymantria monacha larvae feeding on spruce trees.  Oecologia 118: 210-217.

Kruger, E.L., Volin, J.C. and Lindroth, R.L.  1998.  Influences of atmospheric CO2 enrichment on the responses of sugar maple and trembling aspen to defoliation.  New Phytologist 140: 85-94.

Lovelock, C.E., Posada, J. and Winter, K.  1999.  Effects of elevated CO2 and defoliation on compensatory growth and photosynthesis of seedlings in a tropical tree, Copaifera aromatica.  Biotropica 31: 279-287.

Stiling, P., Moon, D.C., Hunter, M.D., Colson, J., Rossi, A.M., Hymus, G.J. and Drake, B.G.  2002.  Elevated CO2 lowers relative and absolute herbivore density across all species of a scrub-oak forest.  Oecologia 10.1007/s00442-002-1075-5.

Stiling, P., Rossi, A.M., Hungate, B., Dijkstra, P., Hinkle, C.R., Knot III, W.M., and Drake, B.  1999.  Decreased leaf-miner abundance in elevated CO2: Reduced leaf quality and increased parasitoid attack.  Ecological Applications 9: 240-244.