02368nas a2200181   4500008004100000022001400041245009000055210006900145300001400214490000800228520178100236100002302017700002102040700002602061700001902087700002102106856005902127       2020                        eng d  a0027-842400aNutrient dilution and climate cycles underlie declines in a dominant insect herbivore0 aNutrient dilution and climate cycles underlie declines in a domi  a7271-72750 v1173 a<p>Evidence for global insect declines mounts, increasing our need to understand underlying mechanisms. We test the nutrient dilution (ND) hypothesis&mdash;the decreasing concentration of essential dietary minerals with increasing plant productivity&mdash;that particularly targets insect herbivores. Nutrient dilution can result from increased plant biomass due to climate or CO2 enrichment. Additionally, when considering long-term trends driven by climate, one must account for large-scale oscillations including El Niño Southern Oscillation (ENSO), North Atlantic Oscillation (NAO), and Pacific Decadal Oscillation (PDO). We combine long-term datasets of grasshopper abundance, climate, plant biomass, and end-of-season foliar elemental content to examine potential drivers of abundance cycles and trends of this dominant herbivore. Annual grasshopper abundances in 16- and 22-y time series from a Kansas prairie revealed both 5-y cycles and declines of 2.1&ndash;2.7%/y. Climate cycle indices of spring ENSO, summer NAO, and winter or spring PDO accounted for 40&ndash;54% of the variation in grasshopper abundance, mediated by effects of weather and host plants. Consistent with ND, grass biomass doubled and foliar concentrations of N, P, K, and Na&mdash;nutrients which limit grasshopper abundance&mdash;declined over the same period. The decline in plant nutrients accounted for 25% of the variation in grasshopper abundance over two decades. Thus a warming, wetter, more CO2-enriched world will likely contribute to declines in insect herbivores by depleting nutrients from their already nutrient-poor diet. Unlike other potential drivers of insect declines&mdash;habitat loss, light and chemical pollution&mdash;ND may be widespread in remaining natural areas.</p>1 aWelti, Ellen, A.R.1 aRoeder, Karl, A.1 ade Beurs, Kirsten, M.1 aJoern, Anthony1 aKaspari, Michael  uhttp://www.pnas.org/lookup/doi/10.1073/pnas.1920012117