Evidence for global insect declines mounts, increasing our need to understand underlying mechanisms. We test the nutrient dilution (ND) hypothesis—the decreasing concentration of essential dietary minerals with increasing plant productivity—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 2.1–2.7%/y. Climate cycle indices of spring ENSO, summer NAO, and winter or spring PDO accounted for 40–54% 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—nutrients which limit grasshopper abundance—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—habitat loss, light and chemical pollution—ND may be widespread in remaining natural areas.

Summary1. Food protein-carbohydrate content has significant consequences for animal survival, growth and reproduction. Rarely, though, is its effect examined over an animal's entire lifetime. 2. In this study, we reared newly hatched caterpillars [Heliothis virescens Fabricus (Lepidoptera: Noctuidae)] on diets containing different protein/carbohydrate (p/c) ratios. We recorded larval survival, time to pupation, pupal mass, eclosion success, time to eclosion and pupal body lipid content. Additionally, for each treatment, we mated eclosed males and females and measured egg production and egg viability. 3. Larval performance (survival to pupation and time to pupation) was similar across all except the two most extreme treatments. In contrast, pupal performance (mass, eclosion success and time to eclosion) was best on diets that were balanced or slightly protein-biased. However, eclosion success differed between sexes. For males, it was best on diets with balanced p/c ratios, while female eclosion was strong across all but the most carbohydrate-biased diet. Pupal body lipid content in both males and females increased as the food p/c ratio decreased. 4. Egg production was best on diets with balanced or slightly protein-biased p/c ratios. 5. We also estimated the effect of food p/c ratio at the population level, using the data generated in this study. Population size was largest on diets with a balanced p/c ratio and declined steadily and strongly as the food p/c ratio became increasingly more imbalanced. 6. Our findings show, for the first time, the effect of food p/c content over an insect herbivore's entire life. Our data indicate that there is a narrow range of p/c ratios that maximize lifetime performance, and for H. virescens, this range coincides with its self-selected p/c ratio.

Abstract. Nitrogen and phosphorus frequently limit terrestrial plant production, but have a mixed record in regulating the abundance of terrestrial invertebrates. We contrasted four ways that Na could interact with an NP fertilizer to shape the plants and invertebrates of an inland prairie. We applied NP and Na to m 2 plots in a factorial design. Aboveground invertebrate abundance was independently co-limited by NaCl and NP, but with +NP plots supporting more individuals. We suggest the disparity arises because NP enhanced plant height by 35% (1 SD) over controls, providing both food and habitat, whereas NaCl provides only food. Belowground invertebrates showed evidence of serial co-limitation, where NaCl additions alone were ineffectual, but catalyzed access to NP. This suggests the increased belowground food availability in NP plots increased Na demand. Na and NP supply rates vary with climate, land use, and with inputs like urine. The co-limitation and catalysis of N and P by Na thus has the potential for predicting patterns of abundance and diversity across spatial scales.

Aim: In ectotherms, gradients of environmental temperature can regulate metabolism, development and ultimately fitness. The thermal adaptation hypothesis assumes that thermoregulation is costly and predicts that more thermally variable environments favour organisms with wider thermal ranges and thermal limits (i.e., critical thermal minima and maxima, CT min and CT max) which track environmental temperatures. We test the thermal adaptation hypothesis at two biological levels of organization, the community and species level. Location: Continental USA.