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Effects of Elevated CO2 on Plant Attacks by Herbivorous Insects
Klaiber, J., Najar-Rodriguez, A.J., Piskorski, R. and Dorn, S. 2013. Plant acclimation to elevated CO2 affects important plant functional traits, and concomitantly reduces plant colonization rates by an herbivorous insect. Planta 237: 29-42.

According to Klaiber et al. (2013), many studies have explored the direct effects of atmospheric CO2 enrichment on plant growth and development, but they add that the effects of elevated CO2 on the behavioral responses of insect pests associated with major food crops have not yet been thoroughly investigated, noting that "the potential effects of plant acclimation to CO2 on host plant colonization by specialized herbivorous insects, which is a key step in the process of plant attack leading to potential yield losses, are largely unknown."

What was done
In light of this dearth of knowledge, Klaiber et al., as they describe it, "used a model system comprised of Brussels sprout (Brassica oleraceae var. gemmifera) and a specialized herbivorous insect, the cabbage aphid (Brevicoryne brassicae) to test for the effects of various periods of exposure to an elevated (2 x ambient) CO2 concentration on key plant functional traits and on host plant location behavior by the insect, assessed as plant colonization rates." And what did their experiment reveal?

What was learned
First off, plant height, leaf number, leaf area, fresh weight, and dry weight all increased in the elevated CO2 treatments with respect to values obtained under ambient conditions, regardless of the length of CO2 exposure (see Table 1 below). Leaf stomatal conductance also benefited at the higher CO2 level, being reduced by nearly 50% by the end of the 10-week experiment.

Table 1. Mean plant parameters measured on Brassica plants grown under ambient (400 ppm) or elevated (800 ppm) CO2 concentrations for 2, 6, and 10 weeks.

With respect to plant/herbivore interactions, the four Swiss scientists say that "doubling the ambient CO2 concentration had a marked effect on plant colonization by winged aphids particularly when plants were exposed to CO2 for longer periods." After an exposure of only 2 weeks, for example, there was no difference in colonization rate, whereas after 6 and 10 weeks "elevated CO2 led to a respective 15 and 26 % reduction of colonization rates" by the cabbage aphid. The authors also note that plant volatile emissions, which have been linked to attracting insects, were significantly reduced at the higher CO2 concentration.

What it means
In light of the above, Klaiber et al. conclude that "in agroecosystems, reduced crop plant colonization by an herbivorous pest insect under elevated CO2 might be advantageous." Indeed, if such amazing CO2-induced benefits are found to extend to other plant species, and there is no reason to suspect they would not, farmers the world over will have one more reason to celebrate the steady rise of atmospheric CO2!

Reviewed 5 June 2013