Too much nitrogen: The paradoxical effects of nutrient ratios and supply rates on an outbreaking insect herbivore

Outbreaks of herbivorous insects may be partly due to the quality and quantity of key host plants.  The Australian plague locust, Chortoicetes terminifera, develops in an environment dominated by perennial Mitchell grass.  Eruptions of C. terminifera, occur following summer rainfall when Mitchell grass remains green for the duration of the locusts’ lifecycle.  The accumulation of lipids to fuel long distance flight, allowing migration to exploit better resources, is a key component of Australian plague locust dynamics.  Chemical analysis suggests nitrogen should be limiting in Mitchell grass.  Paradoxically, due to differential assimilation of protein and carbohydrate, nitrogen is limiting, but not due to its deficiency, rather its excess relative to carbohydrate.  The relative amounts of these macronutrients assimilated by nymphs are influenced by an interaction between nymphal size and plant chemistry.  In the field, as Mitchell grass dries off following rainfall the amounts of water and protein decrease, two factors that negatively affect the amount of carbohydrate assimilated. Thus, although carbohydrate appears to be in excess, it is in fact limiting and as a result, if Mitchell grass dries off prematurely, locusts are unable to develop sufficient fat stores for migration.  This study has demonstrated that to understand the nutritional ecology of herbivores, not only does the nutrient composition of the host plant need to be considered, but also the rate and ratio of supply to the herbivore.  Studies that use single nutrient currencies or correlate nutrient concentration with performance, will fail to recognize critical relationships that explain mechanisms.