Changes To Insect Herbivore Feeding

There have been over 40 studies of the performance of insect herbivores of various types under conditions of elevated CO2.

FIGURE 1 Schematic representation of the effects of elevated CO2 on insects.

The majority of these, over 80%, have been conducted with leaf-chewing insects, especially lepidopteran caterpillars. The most commonly reported change is that food consumption increases as the insects struggle to obtain sufficient nitrogen in their diet. The efficiency of food conversion to insect biomass (conversion efficiency) decreases, probably because of the increased concentration of secondary chemicals, such as tannins, which bind digestive enzymes and render them less effective. Thus, it takes insects much longer to develop, and their final weight is often reduced. Early instars seem to be more susceptible than late instars. Of course such changes in diet could, in theory, be partly offset by the increase in digestibility due to the increased water content. However, at least in the studies done so far, the net outcome of elevated CO2 on herbivorous insect digestibility has been negative.

The responses just outlined may vary somewhat according to the feeding guild of insects involved. Thus, chewing insects, which often digest the whole leaf and encounter both reduced nitrogen levels and increased defensive compounds, are particularly susceptible to changes in nitrogen and phenolics. Insects that feed in a different way may be less susceptible. Phloem and xylem feeders in particular may be less affected by CO2 because they feed on plant sap, which is low in defensive compounds. Seed feeders also may be less affected by increased CO2 because these plants try to maintain high levels of nitrogen in their reproductive parts. In cotton, for example, the C:N ratio of cotton balls is unaffected by elevated CO2 and lepidopterans feeding there are unaffected. The concern is that pest insects could be stimulated to feed on these reproductive parts when the quality of the remainder of the plant decreases, which in turn would increase the pest status of some insects.

Of course CO2 has also the direct effect of increasing temperature via the greenhouse effect, which may stimulate feeding activity because of increased metabolic rate in higher temperatures. Studies on the green peach aphid, Myzus persicae, a pest of many crops, suggest that elevated temperature increases aphid population growth rate and thus the likelihood that aphids will become more important pests in the future. In this case, both elevated CO2 and elevated temperature increased aphid densities in experiments. Since, however, very few experiments have examined both CO2 concentration and temperature in factorial experiments, the generality of the aphid results is unknown. It is also possible that the effects of elevated CO2 and elevated temperature could cancel each other out for other insect species, especially leaf chewers.

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