What was done
The author reviewed recently published scientific literature to summarize current knowledge concerning population responses of herbivorous insects to atmospheric CO2 enrichment.  In this review, the author specifically utilized papers wherein results were obtained from relatively long-term, rather than short-term, CO2 enrichment studies.

What was learned
Of all studies to date (30+), the only herbivorous insects that have exhibited population increases in response to elevated CO2 exposure were those classified as phloem feeders, specifically, aphids.  Chewing insects (leaf chewers and leaf miners), on the other hand, showed either no change or reductions in abundance in response to atmospheric CO2 enrichment.  Oftentimes, population reductions in this feeding guild were accompanied by increased herbivory to account for CO2-induced reductions in leaf nitrogen content.

What it means
As the CO2 content of the air continues to rise, it is likely that herbivorous insects will exhibit differential feeding responses.  In some cases, these responses may lead to changes in herbivore populations.  Phloem feeders, for example, may increase their abundances in a CO2-enriched world, while leaf chewers and miners may experience decreases in population numbers.  Nonetheless, the author acknowledges that "it is not known whether adaptation in insect-plant relations would occur over the decades required for this change [in CO2 concentration] to occur."  Thus, although no one knows for sure what the future may hold for us in this regard, there will probably be little change in the percentage losses of agricultural crops attributable to insect pests as the air's CO2 concentration continues to rise.