Coping with drought: Diapause plasticity in katydid eggs at high, mid, and low elevations

Authors: Srygley, Robert

Submitted to: Ecological Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/11/2019
Publication Date: 6/1/2020
Citation: Srygley, R.B. 2020. Coping with drought: Diapause plasticity in katydid eggs at high, mid, and low elevations. Ecological Entomology. 45(3):485-492. https://doi.org/10.1111/een.12820.
DOI: https://doi.org/10.1111/een.12820

Interpretive Summary: Semi-arid rangelands of the western United States are particularly sensitive to climate change, with extreme droughts projected to increase in frequency and duration in the latter half of the 21st century. These extreme climatic events also impact many insect populations. In the past decade, outbreaks of Mormon crickets occurred in the western U.S. The broad region affected suggests that a climatic event across a large geographic scale promoted rapid population growth of this flightless katydid. A unique adaptation allows some katydid species to prolong embryonic development and remain in an egg bank as a blastoderm for multiple growing seasons (i.e., ‘prolonged diapause’ among other names). We hypothesized that if drought could prolong diapause of Mormon cricket eggs, then it might synchronize embryonic development and hatching after moisture is restored. We compared a high elevation Wyoming population (WY) with two Oregon and Idaho (OR and ID) at lower, drier elevations where we predicted the eggs would be more tolerant of desiccation. We predict that eggs will show the least development in drought during the first growing season, but the undeveloped eggs in the driest treatments will show the greatest development following restoration of moisture and a second growing season. For WY, the two drier treatments had significantly more eggs prolonging development until after the first warm period than the two wetter treatments. Whether those eggs in prolonged diapause developed in the second or subsequent warm periods did not differ among moisture treatments. For OR, significantly fewer embryos developed at the driest treatment compared to the others, and for ID, almost all of the eggs developed irrespective of the moisture treatment. In conclusion, Mormon crickets can delay embryonic development to avoid drought until favorable conditions for growth and hatching are restored. In general, more severe drought delayed development of Mormon cricket embryos, but populations differed in susceptibility to drought. Because undeveloped eggs lose less water than developed embryos, the plastic response of WY appears to have mitigated the effect of drought on egg viability. Unexpectedly, eggs from high elevation were extremely tolerant due to their capacity to postpone development to any one of several more favorable growing seasons. OR also reduced egg loss by prolonging diapause, relative to ID that developed in even the driest condition. Although drought did not result in a concentration of development and hatching, diapause plasticity allowed the insects to cope and await more favorable conditions.

Technical Abstract: Semi-arid rangeland productivity is limited by precipitation, and yet droughts are projected to increase in frequency and duration with unknown impacts on insect populations. Because some katydids can prolong diapause and remain in an egg bank as a blastoderm for multiple growing seasons, could drought prolong diapause and synchronize embryonic development and hatching of Mormon crickets Anabrus simplex after moisture is restored? I compared a high elevation Wyoming population (WY) with two from Oregon and Idaho (OR and ID) at lower, drier elevations where I predicted eggs would be more tolerant of desiccation. I measured the developmental state and water loss after drought treatments and when moisture was restored. The two drier treatments had significantly more WY eggs prolonging diapause until after drought ended than the two wetter treatments. Whether WY eggs in prolonged diapause developed in the second or subsequent warm periods did not differ among drought treatments. For OR, significantly fewer embryos developed at the driest treatment compared to the others, whereas almost all ID eggs developed irrespective of the drought treatment. In conclusion, Mormon crickets can delay embryonic development to avoid drought until favorable conditions for growth are restored. Unexpectedly, eggs from the highest, wettest site were more tolerant because postponing development resulted in less water loss than developed embryos. OR also reduced egg loss by prolonging diapause, relative to ID that developed in even the driest condition. Although drought did not synchronize development and hatching, diapause plasticity allowed insects to cope and await more favorable conditions.