Livestock grazing directs locust outbreaks by altering host plant nitrogen status

Outbreaks and migratory swarms of Oedaleus asiaticus, a dominant Chinese locust, have increased in recent decades. Since the magnitude of outbreaks can be closely associated with heavy livestock grazing, understanding first what the livestock stocking rate threshold is and, second, the mechanisms that direct outbreaks under these circumstances could greatly enhance the ability to sustainably manage rangeland and minimize outbreaks. First, we surveyed locust populations in a large-scale field site manipulated for sheep stocking rate and management type (traditional=grazing every year; mixed=hay making and grazing alternated each year). Locusts were most abundant in fields that were grazed every year with a stocking rate of 9 sheep per hectare and in vegetation patches that were perpetually grazed every year within the 6 sheep per hectare fields. Continuous heavy livestock grazing leads to enhanced erosion, nitrogen (N) depleted soils, and lower plant N content as compared to lightly or ungrazed plots. Thus, second, we tested the hypothesis that plant nutrient content was optimal for locusts in heavily grazed fields. Current paradigms generally assume that increased plant N enhances herbivore performance by relieving N-protein-limitation. In contrast, we found that N-enrichment decreased locust size and viability. Accordingly, this locust preferred plants with low N content – both in paired trials of plants collected from grazed and ungrazed pastures and from control and N-fertilized plots. These results suggest that the dramatic increase in the livestock population of NE China over recent decades and consequent steppe degradation may be promoting locust outbreaks by reducing plant N content.