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Insect Herbivores in a CO2-Enriched Warmer World
Reference
Williams, R.S., Lincoln, D.E. and Norby, R.J. 2003. Development of gypsy moth larvae feeding on red maple saplings at elevated CO2 and temperature. Oecologia 137: 114-122.

What was done
The authors bagged first instar gypsy moth (Lymantria dispar L.) larvae on branches of red maple (Acer rubrum L.) saplings that were entering their fourth year of growth within open-top chambers that were maintained at four sets of CO2/temperature conditions: (1) ambient temperature, ambient CO2, (2) ambient temperature, elevated CO2 (ambient + 300 ppm), (3) elevated temperature (ambient + 3.5C), ambient CO2, and (4) elevated temperature, elevated CO2. Under these conditions they measured several parameters that were required to test their hypothesis that a CO2-enriched atmosphere would lead to reductions in foliar nitrogen concentrations and increases in defensive phenolics that would in turn lead to increases in insect mortality.

What was learned about CO2
The authors' data demonstrated, in their words, "that larvae feeding on CO2-enriched foliage ate a comparably poorer food source than those feeding on ambient CO2-grown plants, irrespective of temperature." They also observed a marginally significant reduction in leaf water due to CO2 enrichment. Nevertheless, they found that "CO2-induced reductions in foliage quality (e.g. nitrogen and water) were unrelated to insect mortality, development rate and pupal weight."

What the CO2 findings mean
In light of what they observed, the authors were forced to conclude that "phytochemical changes resulted in no negative effects on gypsy moth performance." And they say that this observation "contradicted our prediction that alterations in leaf phytochemistry would affect gypsy moth development."

What was learned about temperature
The authors report that "irrespective of CO2 concentration, on average, male larvae pupated 7.5 days earlier and female larvae 8 days earlier at elevated temperature."

What the temperature findings mean
The authors note that anything that prolongs the various development stages of insects potentially exposes them to greater predation and parasitism risk, as noted by Clancy and Price (1987). Consequently, the observed temperature-induced hastening of the insects' development must expose them, if anything, to less predation and parasitism risk.

What it all means
Contrary to what many scientists have long believed, the results of this study clearly indicate that concomitant increases in atmospheric CO2 concentration and temperature need not be deleterious to earth's insect herbivores. In some cases, in fact, they can even be beneficial.

Reference
Clancy, K.M. and Price, P.W. 1987. Rapid herbivore growth enhances enemy attack: sublethal plant defenses remain a paradox. Ecology 68: 733-737.


Reviewed 17 December 2003