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Insects (Other Species) -- Summary
As the air's CO2 content continues to rise, it is important to determine what effect this phenomenon may have on the delicate balance that exists between various plants and the insects that feed upon them.  We treat this subject in some detail with respect to aphids, butterflies and moths in other sections of our website devoted to this general topic, while here we review the results of studies that have been reported for other types of insects.

Docherty et al. (1997), in addition to studying aphids, studied two sap-feeding leafhopper species that were allowed to feed on saplings of beech and sycamore that were grown in glasshouses maintained at atmospheric CO2 concentrations of 350 and 600 ppm.  As far as they could determine, there were no significant effects of the extra CO2 on the feeding and performance characteristics of either leafhopper species.

Two years later, in a review of more than 30 studies published to that point in time, Whittaker (1999) found that leaf chewers and miners showed either no change or actual reductions in abundance in response to atmospheric CO2 enrichment, noting, however, that population reductions in this feeding guild were often accompanied by increased herbivory in response to CO2-induced reductions in leaf nitrogen content.

In a study of yet another type of insect herbivore, Brooks and Whittaker (1999) removed grassland monoliths from the Great Dun Fell of Cumbria, UK - which contained eggs of a destructive xylem-feeding spittlebug - and grew them in glasshouses maintained for two years at atmospheric CO2 concentrations of 350 and 600 ppm.  During the course of their experiment, two generations of the xylem-feeding insect were produced; and in each case, elevated CO2 reduced the survival of nymphal stages by an average of 24%.  The two researchers suggest that this reduction in survival rate may have been caused by CO2-induced reductions in stomatal conductance and transpirational water loss, which may have reduced xylem nutrient-water availability.  Whatever the mechanism, the results of this study bode well for the future survival of these species-poor grasslands as the air's CO2 content continues to rise.

Noting that increases in the air's CO2 content typically lead to greater decreases in the concentrations of nitrogen and, therefore, protein in the foliage of C3 as compared to C4 grasses, Barbehenn et al. (2004b) say "it has been hypothesized that herbivores will disproportionately increase their feeding damage on C3 plants to compensate for the larger changes in C3 plants in elevated CO2 (Lincoln et al., 1984, 1986; Lambers, 1993)."  To test this hypothesis, the authors grew Italian ryegrass (a common C3 pasture grass) and sideoats gramma (a native C4 rangeland grass) in chambers maintained at either the ambient atmospheric CO2 concentration of 370 ppm or the doubled CO2 concentration of 740 ppm for two months, after which grasshopper (Melanoplus sanguinipes) nymphs that had been reared to the fourth instar stage were allowed to feed upon the grasses' foliage.  As expected, foliage protein concentration decreased much more in the C3 grass than in the C4 grass (22% vs. 7%) when the grasses were grown in CO2-enriched air.  However, and "contrary to the hypothesis that insect herbivores will increase their feeding rates disproportionately in C3 plants under elevated atmospheric CO2," Barbehenn et al. report that "M. sanguinipes did not significantly increase its consumption rate when feeding on the C3 grass grown under elevated CO2," suggesting that this observation implies that "post-ingestive mechanisms enable these grasshoppers to compensate for variable nutritional quality in their host plants," and that some of these post-ingestive responses may include "changes in gut size, food residence time, digestive enzyme levels, and nutrient metabolism (Simpson and Simpson, 1990; Bernays and Simpson, 1990; Hinks et al., 1991; Zanotto et al., 1993; Yang and Joern, 1994a,b)."  In addition, their data indicate that, if anything, M. sanguinipes growth rates were increased, perhaps by as much as 12%, when they fed upon the C3 foliage that had been produced in the CO2-enriched air.

Consequently, and just as was found in the study of Barbehenn et al. (2004a), the CO2-induced decrease in leaf protein concentration observed in this study did not induce an increase in foliage consumption in the C3 plant studied, nor did it reduce the growth rate of the herbivore studied.  With respect to this finding, as well as the similar findings of others, they thus state that "although compensatory feeding was commonly observed in early studies [of this subject], the absence of compensatory feeding on C3 plants grown under elevated CO2 has since been observed frequently among herbivorous insects (Bezemer and Jones, 1998)," which suggests that the latter response may ultimately be found to be the more common of the two.

Finally, we report the results of two studies that have nothing to do with CO2, but with global warming, which climate-alarmists claim is caused by elevated levels of this trace atmospheric constituent, and which they say will be so fast and furious that many plants and animals (including insects) will not be able to migrate rapidly enough towards cooler regions of the globe to avoid extinction.

In the first of these studies, Thomas et al. (2001) demonstrated that in response to regional warming in the British Isles over the last two decades of the 20th century, there was an unusually rapid northward expansion of the ranges of two species of cricket: the long-winged cone-head and Roesel's bush cricket.  The reason for this increased rate of warming-induced range expansion, according to them, was that the crickets produced "increased fractions of longer-winged (dispersive) individuals in recently founded populations" that "resulted in about 3- to 15-fold increases in expansion rates, allowing these insects to cross habitat disjunctions that would have represented major or complete barriers to dispersal before the expansions started."

In the second such study, Hickling et al. (2005) analyzed changes in the northern and southern range boundaries of 37 non-migratory British Odonata (dragonfly and damselfly) species - 4 of which have northern ranges, 24 of which have southern ranges, and 9 of which are ubiquitous - between the two 10-year periods 1960-70 and 1985-95.  This work revealed that all but two of the 37 species increased the sizes of their ranges between the two 10-year periods, leading Hickling et al. to report that their "findings that species are shifting northwards faster at their northern range margin than at their southern range margin, are consistent with the results of Parmesan et al. (1999)," adding that "this could suggest that species at their southern range margins are less constrained by climate than by other factors."  We agree, noting that this is the primary thesis of our major report The Specter of Species Extinction: Will Global Warming Decimate Earth's Biosphere?  Thus, rather than leading to range reductions and a subsequent massive extinction of species, as has been claimed by many climate alarmists to be lurking just around the corner, so to speak, global warming, if it continues for some time and its elevated warmth is maintained, will in all likelihood lead to most of earth's species expanding their ranges and gaining even stronger footholds on the planet.

In conclusion, and in consideration of the implications of the various phenomena described in this summary, it would appear that both CO2-induced and warming-induced changes in the physical characteristics and behavior patterns of a diverse assemblage of insects portend nothing but good for the biosphere in the years and decades to come, which is just the opposite of what the world's climate alarmists continually claim.  CO2-induced changes in insect pests will not give them an undue advantage over their host plants, while warming-induced changes will help them to actually expand their ranges and avoid extinction.

References
Barbehenn, R.V., Karowe, D.N. and Spickard, A.  2004a.  Effects of elevated atmospheric CO2 on the nutritional ecology of C3 and C4 grass-feeding caterpillars.  Oecologia 140: 86-95.

Barbehenn, R.V., Karowe, D.N. and Chen, Z.  2004b.  Performance of a generalist grasshopper on a C3 and a C4 grass: compensation for the effects of elevated CO2 on plant nutritional quality.  Oecologia 140: 96-103.

Bernays, E.A. and Simpson, S.J.  1990.  Nutrition.  In: Chapman, R.F. and Joern, A. (Eds.).  Biology of Grasshoppers.  Wiley, New York, NY, pp. 105-127.

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.

Brooks, G.L. and Whittaker, J.B.  1999.  Responses of three generations of a xylem-feeding insect, Neophilaenus lineatus (Homoptera), to elevated CO2Global Change Biology 5: 395-401.

Docherty, M., Wade, F.A., Hurst, D.K., Whittaker, J.B. and Lea, P.J.  1997.  Responses of tree sap-feeding herbivores to elevated CO2Global Change Biology 3: 51-59.

Hickling, R., Roy, D.B., Hill, J.K. and Thomas, C.D.  2005.  A northward shift of range margins in British Odonata.  Global Change Biology 11: 502-506.

Hinks, C.R., Cheeseman, M.T., Erlandson, M.A., Olfert, O. and Westcott, N.D.  1991.  The effects of kochia, wheat and oats on digestive proteinases and the protein economy of adult grasshoppers, Malanoplus sanguinipesJournal of Insect Physiology 37: 417-430.

Lambers, H.  1993.  Rising CO2, secondary plant metabolism, plant-herbivore interactions and litter decomposition. Theoretical considerations.  Vegetatio 104/105: 263-271.

Lincoln, D.E., Sionit, N. and Strain, B.R.  1984.  Growth and feeding response of Pseudoplusia includens (Lepidoptera: Noctuidae) to host plants grown in controlled carbon dioxide atmospheres.  Environmental Entomology 13: 1527-1530.

Lincoln, D.E., Couvet, D. and Sionit, N.  1986.  Responses of an insect herbivore to host plants grown in carbon dioxide enriched atmospheres.  Oecologia 69: 556-560.

Parmesan, C., Ryrholm, N., Stefanescu, C., Hill, J.K., Thomas, C.D., Descimon, H., Huntley, B., Kaila, L., Kullberg, J., Tammaru, T., Tennent, W.J., Thomas, J.A. and Warren, M.  1999.  Poleward shifts in geographical ranges of butterfly species associated with regional warming.  Nature 399: 579-583.

Simpson, S.J. and Simpson, C.L.  1990.  The mechanisms of nutritional compensation by phytophagous insects.  In: Bernays, E.A. (Ed.).  Insect-Plant Interactions, Vol. 2. CRC Press, Boca Raton, FL, pp. 111-160.

Thomas, C.D., Bodsworth, E.J., Wilson, R.J., Simmons, A.D., Davies, Z.G., Musche, M. and Conradt, L.  2001.  Ecological and evolutionary processes at expanding range margins.  Nature 411: 577-581.

Whittaker, J.B.  1999.  Impacts and responses at population level of herbivorous insects to elevated CO2European Journal of Entomology 96: 149-156.

Yang, Y. and Joern, A.  1994a.  Gut size changes in relation to variable food quality and body size in grasshoppers.  Functional Ecology 8: 36-45.

Yang, Y. and Joern, A.  1994b.  Influence of diet quality, developmental stage, and temperature on food residence time in the grasshopper Melanoplus differentialisPhysiological Zoology 67: 598-616.

Zanotto, F.P., Simpson, S.J. and Raubenheimer, D.  1993.  The regulation of growth by locusts through post-ingestive compensation for variation in the levels of dietary protein and carbohydrate.  Physiological Entomology 18: 425-434.

Last updated 28 December 2005