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Heat Dosage and Oviposition Depth Influence Egg Mortality of Two Common Rangeland Grasshopper Species

David H. Branson, Lance T. Vermeire
Rangeland ecology & management 2013 v.66 no.1 pp. 110-113
Aulocara elliotti, egg masses, eggs, fire intensity, grasshoppers, heat transfer, heat treatment, mortality, population density, rangelands, wildfires
Rangeland fire is a common naturally occurring event and management tool, with the amount and structure of biomass controlling transfer of heat belowground. Temperatures that grasshopper eggs are exposed to during rangeland fires are mediated by species-specific oviposition traits. This experiment examined egg mortality in two slant-faced grasshopper species with differing oviposition traits, namely Aulocara elliotti (Thomas) and Opeia obscura (Thomas). We hypothesized that A. elliotti egg mortality would increase with fire intensity because the shallow egg location below the soil surface would result in exposure to higher temperatures, and that the deeper O. obscura eggs would not be affected by fire intensity. Fire intensity did not significantly affect the mortality of O. obscura eggs, with very low mortality in all treatments. Fire intensity significantly affected mortality of A. elliotti eggs, which are laid in shallow egg pods with the midpoint of the egg clutch at a depth of ∼ 0.825cm. Aulocara elliotti egg mortality increased with higher levels of heat application, with 79% egg mortality in the 4 500 kg · ha−1 heat treatment. Heat effects on A. elliotti egg mortality were similar to those previously observed for another shallow-egg-laying species. Limited research has examined if rangeland fires reduce population densities of specific economically important grasshopper species. The results from this experiment indicate that grasshopper species with the midpoint of the egg pod less than 1cm below the surface are likely in general to be vulnerable to fire-induced egg mortality during rangeland fires.