Elevational and latitudinal changes in cold tolerance of nymph and adult Mormon crickets Anabrus simplex (Orthoptera: Tettigoniidae)

Authors: Srygley, Robert

Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/12/2021
Publication Date: 6/1/2021
Citation: Srygley, R.B. 2021. Elevational and latitudinal changes in cold tolerance of nymph and adult Mormon crickets Anabrus simplex (Orthoptera: Tettigoniidae). Environmental Entomology. 50(3):699-705. https://doi.org/10.1093/ee/nvab009.
DOI: https://doi.org/10.1093/ee/nvab009

Interpretive Summary: 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. In general, insects from high elevations or high latitudes should be more resistant to colder temperatures. Mormon crickets occupy desert and montane regions across a broad elevational range from sea-level to 3000 m and a broad latitudinal range from southwestern U.S. to Canada. Population declines have been attributed to late freezing events, but winter hatching suggests they may also be cold tolerant. Research on cold tolerance in Mormon cricket populations is important to understanding population growth and predicting outbreaks. Minimum 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. For example, half of the nymphs died after 6 h exposure to -10°C for AZ relative to -7°C for WY. 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. Half of the OR adults died after 24 h exposure to -7°C relative to -5°C for WY. In addition, third instar nymphs from mid-elevation ID were the most susceptible to cold exposure. Lower lethal temperature is an important factor in understanding the rapid decline of Mormon crickets from the landscape following outbreaks.

Technical 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 U.S. to Canada and a broad elevational range from near sea-level to 3000 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.