Heat dosage and oviposition depth influence egg mortality of two common rangeland grasshopper species

Authors: Branson, David - Dave; Vermeire, Lance

Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/12/2012
Publication Date: 1/2/2013
Citation: Branson, D.H., Vermeire, L.T. 2013. Heat dosage and oviposition depth influence egg mortality of two common rangeland grasshopper species. Rangeland Ecology and Management. 66(1):110-113.

Interpretive Summary: Grasshoppers are often the most economically important insects in North American (N.A.) rangeland systems, with rangeland fire a common naturally occurring event and management tool. Rangeland grasshopper species oviposit eggs at depths ranging from just below to greater than 5 cm below the soil surface, with the size and orientation of egg pods also varying greatly. As a result, temperatures eggs are exposed to during rangeland fires are mediated by species specific oviposition traits. To enhance predictions of grasshopper population responses following fire, the importance of fire mediated egg mortality needs to be examined in multiple species. By examining a range of realistic fire intensities, predictions can be made regarding the fuel load associated with egg mortality. The two Gomphocerine grasshoppers examined are gramnivorous egg-overwintering species, with Aulocara elliotti being a very economically important species that is frequently the dominant species in rangeland grasshopper outbreaks.The results from this experiment indicate that that the subset of rangeland grasshopper species in western North America with midpoint egg pod depths of less than 1 cm have the potential to have their populations managed through burning, particularly if they preferentially oviposit near vegetative cover.

Technical 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 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, 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. Fire intensity significantly affected mortality of A. elliotti eggs, which lays shallow egg pods with an average egg depth of ~0.825 cm. Aulocara elliotti egg mortality increased with higher levels of heat application. Heat effects on A. elliotti egg mortality were similar to those previously observed for another shallow egg laying species. Little 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 midpoint egg pod depths of less than 1 cm are likely in general to be vulnerable to fire-induced egg mortality during rangeland fires.