Elevational and Latitudinal Changes in Cold Tolerance of Nymph and Adult Mormon Crickets Anabrus simplex (Orthoptera: Tettigoniidae).

Abstract

Insects that hatch in winter and early spring in desert and montane regions are likely to encounter extreme weather events, including precipitous drops in temperature. The susceptibility of insects to exposure to subzero temperatures is predicted to decrease with increasing latitude or elevation. Mormon crickets occur over a broad latitudinal range from southwestern United States to Canada and a broad elevational range from near sea-level to 3,000 m. Population declines have been attributed to late freezing events, but winter hatching suggests they may also be cold tolerant. Lower lethal temperature of high elevation populations in low latitude Arizona (AZ) and high latitude Wyoming (WY) was measured by exposing nymphs and adults to 6 h or 24 h of subzero temperature. From similar latitude, WY was compared with mid-elevation Idaho (ID) and low elevation Oregon (OR) populations. Contrary to the prediction, lethal temperature of third instar nymphs was lower in AZ than in the more northerly populations. Consistently, AZ was more tolerant of cold in early nymphal instars relative to populations from higher latitude. Early hatching at lower latitudes might increase the risk of early instars experiencing a severe cold snap relative to nymphs at high latitudes. Also, contrary to prediction, the lethal temperature of adults increased with elevation, whereas third instar nymphs from mid-elevation ID were the most susceptible to cold exposure. Cold tolerance in immature and mature stages is more likely to be uncoupled when life stages do not coincide, as with Mormon crickets.