Reference
Adler, L.S., De Valpine, P., Harte, J. and Call, J. 2007. Effects of long-term experimental warming on aphid density in the field. Journal of the Kansas Entomological Society 80: 156-168.
Background
The authors note that "global warming is generally predicted to increase the intensity of herbivore pressure on plants," but that "support for this prediction often comes from short-term studies, or studies conducted in controlled laboratory settings."
What was done
Introducing considerably more realism into the study of the subject, Adler et al. examined the effects of long-term experimental warming on an aphid-sagebrush (Obtusicauda coweni-Artemisia tridentata) interaction out-of-doors in the field at the Rocky Mountain Biological Laboratory, Colorado, USA, where five of ten 3-m x 10-m plots in an ungrazed montane meadow were warmed by overhead infrared heaters that had provided a continuous heat flux of 22 W/m2 to the plots ever since 1993. Working at this facility for a period of three consecutive years (1996-1998), they determined how warming affected aphid density, while additional observations, manipulative experiments and chemical analyses were used to explore some of the mechanisms that might possibly mediate potential effects of warming on aphid density.
What was learned
The four researchers report that "in no year did we find support for the prediction that warming increased aphid abundance or population growth." In fact, they say they found that "warming decreased aphid density on sagebrush in one year, tended to decrease aphids in a second year, and had no effect in a third year." In the first of these years (1997), in fact, they say "there were over 3 times as many aphids per stalk in control compared to warmed plots," and that "the proportion of stems infested was almost twice as high." Furthermore, they observed that "in enclosures that excluded predators, warming decreased aphid population growth by an amount consistent with observed field density differences," and in a separate snow-manipulation experiment in unheated plots, they found that "the timing of snowmelt did not affect aphid density."
What it means
In light of their several observations, Adler et al. concluded that "long-term studies within a natural community context may provide counterexamples to the prediction that warming will increase herbivore pressure on plants," just as it did in their study.