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

Abstract

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 ~0.825
cm. Aulocara elliotti egg mortality increased with higher levels of heat application, with 79% egg mortality in the 4 500 kg ha1
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 1 cm below the surface are likely in general to be vulnerable to fire-induced egg mortality during rangeland fires.

Key Words: Aulocara elliotti, bigheaded grasshopper, fire, grassland, obscure grasshopper, Orthoptera