Background
Reviewing the pertinent scientific literature in an attempt to determine if plant-feeding insects would become more common, less common or remain essentially unchanged in numbers in a high-CO2 environment, Lawton (2000) concluded it was impossible to predict even the direction of likely change.  Three years later, Newman broaches the same question with respect to cereal aphids.

What was done
The author reviews what is known, and not known, about aphid responses to concurrent increases in atmospheric CO2 concentration and air temperature.  He also investigates the subject via the aphid population model of Newman et al. (2003).

What was learned
Newman's literature review and model analysis lead him to conclude that when the air's CO2 concentration and temperature are both elevated, "aphid population dynamics will be more similar to current ambient conditions than expected from the results of experiments studying either factor alone."  And if that sounds ambiguous, it is.  We can only draw the general conclusion, according to Newman, that "insect responses to CO2 are unlikely to all be in the same direction."  Nevertheless, he says "the lack of a simple common phenomenon does not deny that there is some overriding generality in the responses by the system."  It's just that we don't yet know what that overriding generality is.

What it means
Here is Newman's perspective: "I do not believe that more similar studies are going to yield the generality we seek.  At least in the case of aphids, and probably phytophagous insects in general, the variability in the direction of change has one or more causes that we must identify.  More experiments manipulating only CO2 will not help elucidate these.  We now need to manipulate CO2, while simultaneously manipulating nutrients, drought stress, temperature, humidity, etc.  In the presence of interactions, it will not be sufficient to simply hold these factors constant - the interactions will obscure the generality."

References
Lawton, J.H.  2000.  Community Ecology in a Changing World.  Ecology Institute, Luhe, Germany.

Newman, J.A., Gibson, D.J., Parsons, A.J. and Thornley, J.H.M.  2003.  How predictable are aphid population responses to elevated CO2?  Journal of Animal Ecology 72: 556-566.