Preference and performance in plant-herbivore interactions across latitude

Background/Question/Methods Theory and data suggest that high-latitude plants are more palatable to herbivores than low-latitude conspecifics. Does increased palatability of plants lead to increased performance of herbivores at high latitude? We used 30 salt marsh sites along the US Atlantic Coast (Florida to Maine) to examine whether plant quality for herbivores increases with latitude, and whether herbivores show local adaptation to plants from their own geographic regions. This study, including both field surveys and laboratory experiments, focused on 3 common plant species (Spartina alterniflora, Solidago sempervirens, Iva frutescens) and 6 associated herbivores (2 aphid, 2 grasshopper, 1 beetle, and 1 planthopper species).

Results/Conclusions Field surveys suggested that high-latitude plants should also have been of higher quality to herbivores. Leaf C:N ratio in all 3 plant species decreased toward higher latitudes, consistent with higher leaf chlorophyll and nitrogen contents at high latitudes. Leaf toughness decreased toward higher latitudes in 1 species, but not in the other two. The body size of 4 herbivore species increased with latitude, consistent with high-latitude leaves being of higher quality, while 2 grasshopper species showed the opposite pattern, likely due to life-history constraints. In the laboratory, the geographic region from which plants were collected affected herbivore performance for 4 (or 5) out of 6 herbivore species. In particular, high-latitude plants supported better herbivore performance in two sucking (aphid, planthopper) and two chewing (beetle, grasshopper) species, and tended to do so in a third chewing herbivore (grasshopper). The geographic region from which herbivores were collected affected herbivore performance in all 6 species; however, the pattern was mixed. We found no evidence for local adaptation by herbivores to plants from their own geographic region. In conclusion, our results suggest that more-palatable plants at high latitudes support better herbivore growth, and geographic origin of either plants or herbivores can affect herbivore performance.