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Effects of Elevated CO2 on Aphid-Infested and Non-Infested Wheat
Chen, F.J., Wu, G. and Ge, F.  2004.  Impacts of elevated CO2 on the population abundance and reproductive activity of aphid Sitobion avenae Fabricius feeding on spring wheat.  JEN 128: 723-730.

What was done
The authors grew spring wheat (Triticum aestivum L. cv. Kehan 50) from seed to maturity in high-fertility well-watered pots out-of-doors in open-top chambers (OTCs) maintained at atmospheric CO2 concentrations of 370, 550 and 750 ppm.  Approximately two months after seeding, 20 apterous adult aphids (Sitobion avenae) from an adjacent field were placed upon the wheat plants of each of 25 pots in each OTC, while 15 pots were left as controls; and at subsequent 5-day intervals, both apterous and alate aphids were counted.  Then, about one month later, 10 alate morph fourth instar nymphs were introduced onto the plants of each of nine control pots; and for the next two weeks the number of offspring laid on those plants were recorded and removed daily to measure reproductive activity.  Last of all, at the end of the study, the wheat plants were harvested and their various growth responses determined.

What was learned
Abundances of the introduced aphid populations increased after infestation, peaked during the grain-filling stage, and then declined a bit as the wheat matured.  On the final day of measurement, aphids in the 550-ppm treatment were 32% more numerous than those in ambient air, while aphids in the 750-ppm treatment were 50% more numerous.  Alate aphids also produced more offspring on host plants grown in elevated CO2: 13% more in the 550-ppm treatment and 19% more in the 750-ppm treatment.

As for the wheat plants, Chen et al. report that "elevated CO2 generally enhanced plant height, aboveground biomass, ear length, and number of and dry weight of grains per ear, consistent with most other studies."  With respect to aboveground biomass, for example, the 550-ppm treatment displayed an increase of 36%, while the 750-ppm treatment displayed an increase of 50%, in the case of both aphid-infested and non-infested plants.

What it means
Chen et al. report that "aphid infestation caused negative effects on all the plant traits measured ... but the negative effects were smaller than the positive effects of elevated CO2 on the plant traits."  Hence, they conclude that "the increased productivity occurring in plants exposed to higher levels of CO2 more than compensate for the increased capacity of the aphids to cause damage."

In this experiment, therefore, we have a situation where both the plant and the insect that feeds on it were simultaneously benefited by the applied increases in atmospheric CO2 concentration.  In other words, the CO2-induced responses resulted in a win-win situation with no loser; both plant and insect profited.

Reviewed 30 March 2005