Kerslake et al. (1998) grew five-year-old heather (Calluna vulgaris) plants collected from a Scottish moor in open-top chambers maintained at atmospheric CO2 concentrations of 350 and 600 ppm.  At two different times during the study, larvae of the destructive winter moth Operophtera brumata -- whose outbreaks periodically cause extensive damage to heather moorland -- were allowed to feed upon current-year shoots.  Feeding upon the high-CO2-grown foliage did not affect larval growth rates, development or final pupal weights; neither was moth survivorship significantly altered.  Hence, the authors concluded that their study "provides no evidence that increasing atmospheric CO2 concentrations will affect the potential for outbreak of Operophtera brumata on this host."  What it did show, however, was a significant CO2-induced increase in heather water use efficiency.

Newman et al. (1999) inoculated tall fescue (Festuca arundinacea) plants growing in open-top chambers maintained at atmospheric CO2 concentrations of 350 and 700 ppm with bird cherry-oat aphids (Rhopalosiphum padi).  After nine weeks, the plants growing in the CO2-enriched air had experienced a 37% increase in productivity and were covered with far fewer aphids than the plants growing in ambient air.  The result, therefore, was a win for the favored plants and a loss for the destructive insects.

Goverde et al. (1999) collected four genotypes of Lotus corniculatus near Paris and grew them in controlled environment chambers kept at atmospheric CO2 concentrations of 350 and 700 ppm.  Larvae of the Common Blue Butterfly (Polyommatus icarus) that were allowed to feed upon the foliage produced in the CO2-enriched air ate more, grew larger and experienced shorter development times than larvae feeding on the foliage produced in the ambient-air treatment, suggesting that this butterfly species will likely become ever more robust and plentiful as the air's CO2 content continues to rise.

Brooks and Whittaker (1999) removed grassland monoliths containing eggs of the xylem-feeding spittlebug Neophilaenus lineatus from the UK's Great Dun Fell in Cumbria and placed them in glasshouses maintained at atmospheric CO2 concentrations of 350 and 600 ppm for two years.  Survival of the spittlebug's nymphal states was reduced by 24% in both of the generations produced in their experiment, suggesting that this particular insect will likely cause less tissue damage to the plants of this species-poor grassland in a CO2-enriched world of the future.

Joutei et al. (2000) grew bean (Phaseolus vulgaris) plants in controlled environments kept at atmospheric CO2 concentrations of 350 and 700 ppm, to which they introduced the destructive agricultural mite Tetranychus urticae, observing that female mites produced 34% and 49% less offspring in the CO2-enriched chambers in their first and second generations, respectively.  This CO2-induced reduction in the reproductive success of this invasive insect, which negatively affects more than 150 crop species worldwide, bodes well for mankind's ability to grow the food we will need to feed our growing numbers in the years ahead.

In a somewhat different experiment, Peters et al. (2000) fed foliage derived from FACE plots of calcareous grasslands of Switzerland (maintained at 350 and 650 ppm CO2) to terrestrial slugs, finding they exhibited no preference with respect to the CO2 treatment from which the foliage was derived.  Also, in a study that targeted no specific insect pest, Castells et al. (2002) found that a doubling of the air's CO2 content enhanced the total phenolic concentrations of two Mediterranean perennial grasses (Dactylis glomerata and Bromus erectus) by 15% and 87%, respectively, which compounds tend to enhance plant defensive and resistance mechanisms to attacks by both herbivores and pathogens.

Finally, within a still more different context, Coviella and Trumbel (2000) determined that toxins produced by Bacillus thuringiensis (Bt), which are applied to crop plants by spraying as a means of combating various crop pests, were "more efficacious" in cotton grown in an elevated CO2 environment than in ambient air, which is a big plus for modern agriculture.  In addition, Coviella et al. (2000) determined that "elevated CO2 appears to eliminate differences between transgenic [Bt-containing] and nontransgenic plants for some key insect developmental/fitness variables including length of the larval stage and pupal weight," which could prove to be a big plus for nature in the event of inadvertent Bt gene transference to wild relatives of transgenic crop lines.

In summary, the majority of evidence that has been accumulated to date suggests that rising atmospheric CO2 concentrations may reduce the frequency and severity of pest outbreaks that are detrimental to agriculture, while not seriously impacting herbivorous organisms found in natural ecosystems that are normally viewed in a more favorable light.

References
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.

Castells, E., Roumet, C., Penuelas, J. and Roy, J.  2002.  Intraspecific variability of phenolic concentrations and their responses to elevated CO2 in two mediterranean perennial grasses.  Environmental and Experimental Botany 47: 205-216.

Coviella, C.E. and Trumble, J.T.  2000.  Effect of elevated atmospheric carbon dioxide on the use of foliar application of Bacillus thuringiensis.  BioControl 45: 325-336.

Coviella, C.E., Morgan, D.J.W. and Trumble, J.T.  2000.  Interactions of elevated CO2 and nitrogen fertilization: Effects on production of Bacillus thuringiensis toxins in transgenic plants.  Environmental Entomology 29: 781-787.

Goverde, M., Bazin, A., Shykoff, J.A. and Erhardt, A.  1999.  Influence of leaf chemistry of Lotus corniculatus (Fabaceae) on larval development of Polyommatus icarus (Lepidoptera, Lycaenidae): effects of elevated CO2 and plant genotype.  Functional Ecology 13: 801-810.

Joutei, A.B., Roy, J., Van Impe, G. and Lebrun, P.  2000.  Effect of elevated CO2 on the demography of a leaf-sucking mite feeding on bean.  Oecologia 123: 75-81.

Kerslake, J.E., Woodin, S.J. and Hartley, S.E.  1998.  Effects of carbon dioxide and nitrogen enrichment on a plant-insect interaction: the quality of Calluna vulgaris as a host for Operophtera brumata.  New Phytologist 140: 43-53.

Newman, J.A., Gibson, D.J., Hickam, E., Lorenz, M., Adams, E., Bybee, L. and Thompson, R.  1999.  Elevated carbon dioxide results in smaller populations of the bird cherry-oat aphid Rhopalosiphum padi.  Ecological Entomology 24: 486-489.

Peters, H.A., Baur, B., Bazzaz, F. and Korner, C.  2000.  Consumption rates and food preferences of slugs in a calcareous grassland under current and future CO2 conditions.  Oecologia 125: 72-81.