Bergmann’s Rule and latitudinal variation in herbivore body size

Bergmann’s rule states that body size tends to increase with latitude.  Variation in body size across latitude could be caused by plastic responses to climate or food quality, or by genetic differences.  To better understand variation in herbivore body size across latitude, we studied 3 plant species and 6 associated herbivore species from 16 salt marsh sites along the U.S. Atlantic Coast, from Florida to Maine.  We tested two hypotheses. 1) In the field, herbivores will be bigger at high latitudes, consistent with Bergmann’ rule.  2)  In the laboratory, when fed with host plants from all latitudes in a common-garden experiment, herbivores from high latitudes will perform better than those from low latitudes.  In the field, the tibia length of aphids and beetles (an indicator of body size) increased toward high latitudes, supporting hypothesis 1.  In contrast, grasshopper body size decreased toward high latitudes.  In the laboratory, aphids and beetles from high latitudes grew faster or larger (regardless of host plant latitude of origin), supporting hypothesis 2.  In contrast, grasshoppers from high latitude performed worse than grasshoppers from low latitudes.  Our results suggest that geographic variation in herbivore body size has a strong genetic component.  Phenological constraints may prevent grasshoppers from following Bergmann’s rule.