Elevational changes in Mormon cricket life histories: Minimum temperature for nymphal growth declines with elevation

Authors: Srygley, Robert

Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/20/2020
Publication Date: 2/1/2021
Citation: Srygley, R.B. 2020. Elevational changes in Mormon cricket life histories: Minimum temperature for nymphal growth declines with elevation. Environmental Entomology. 50(1):167-172. https://doi.org/10.1093/ee/nvaa151.
DOI: https://doi.org/10.1093/ee/nvaa151

Interpretive Summary: As mean temperature and the duration of the growing season decline with elevation, growth of immature insects should be faster and initiate at a lower temperature in order to complete development prior to season’s end. Although flightless, Mormon crickets are notorious for migrating in large aggregations across broad spatial and elevational distances which could limit local adaptation. In addition, selection to be active at cooler temperatures might compromise selection to maximize growth rate. I measured growth rate in controlled environments for nymphs from three populations that were similar in latitude but varied in elevation 30 fold. Between 22-30°C, growth rate increased with temperature. Although the base temperature declined with elevation, the degree days remained the same among populations. As a result, for any given temperature, growth rate increased with elevation, indicating that selection to initiate growth at cooler temperatures did not compromise growth rate. Body mass did not differ between the populations at low and mid-elevations, whereas the high elevation population had smaller adults. For pest management, a base temperature from mid-elevation of 15.3°C (60°F) and growing degree days of 509 (equivalent to 916 Fahrenheit-based degree days) are reasonable estimates for applications from sea level to 2700 m (9000 feet).

Technical Abstract: As mean temperature and the duration of the growing season decline with elevation, growth of immature insects should initiate at a lower temperature but it should also be faster to complete development prior to season’s end. Although flightless, Mormon crickets migrate in large aggregations across broad spatial and elevational distances which might limit adaptations to local environments. In addition, selection to be active at cooler temperatures might limit selection to maximize growth rate. I measured growth rate in controlled environments for nymphs from three populations that vary in altitude (87-2688 m) but are similar in latitude (43.2-45.7°N). Growth rate increased significantly with mean temperature between 22-30°C. The intercept of the regression of growth rate on temperature increased with elevation, whereas the slope did not change significantly. For any given temperature, growth rate increased with elevation, which suggests that selection to initiate growth at cooler temperatures did not compromise growth rate. Body mass did not differ between the two lower elevations, whereas the highest elevation population had smaller hatchlings and adults. Base temperature declined with elevation (0.7°C per 1000 m), and the degree days were 509 across all elevations. For pest management, a base temperature from mid elevation of 15.3°C (60°F) and growing degree days of 509 (equivalent to 916 Fahrenheit-based degree days) are reasonable estimates for applications from sea level to 2700 m (9000 feet).