Author
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FIELDING, DENNIS |
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Submitted to: Journal of Insect Science
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 10/3/2005 Publication Date: 4/7/2006 Citation: Fielding, D.J. 2006. Optimal diapause strategies of a grasshopper, Melanoplus sanguinipes. 16pp. Journal of Insect Science 6:02, available online: insectscience.org/6.02 Interpretive Summary: To prevent eggs from hatching at the wrong time, grasshopper embryos enter diapause upon reaching a certain stage of development. After overwintering, diapause wears off and the eggs are ready to hatch in the spring. Many studies have been conducted with various insects to determine the best time to switch from direct development to diapause. There are other aspects of diapause, however, that are important for understanding the life cycle of insects. In addition to the time of year at which females begin producing diapause eggs, other traits may be important: the ability to skip diapause if the pre-diapause embryo overwinters, and the age at which the embryo stops growing and enters diapause. Diapause traits were examined in two populations of the migratory grasshopper, one group from Idaho and the others from Alaska. Alaskan populations diapaused at a late stage of development and could not skip diapause even if the pre-diapause embryos were chilled for 90 days. The population from Idaho also diapaused at a late stage of development, but were able to skip diapause if chilled before entering diapause. These results explain how eggs of Alaskan grasshoppers overwinter twice. The results also explain patterns of emergence in the spring. Alaskan embryos will be at a late stage of development, in the second spring and hatch in synchrony. In contrast, the Idaho population will be in various stages of development in spring, and hatching will be spread out over a longer time. This wide range in hatching times makes it more difficult to properly time control operations. Technical Abstract: Previous analytical studies of diapause in insects have most often focused on the timing of the switch from non-diapausing to diapausing offspring in bivoltine populations. Other traits, such the stage at which diapause begins, and whether diapause is facultative or obligate, may alter the fitness of different diapause strategies. Many species of grasshoppers (Orthoptera: Acrididae), including Melanoplus sanguinipes F. overwinter as eggs. Various studies from a wide geographical range have reported that embryological diapause in this species is under maternal control, may be obligate or facultative (i.e., may be averted by cold temperature treatment of pre-diapause embryos), and that embryos may enter diapause at different ages. Diapause traits were examined in two populations of M. sanguinipes from very different environments: Alaska and Idaho, USA. Populations from Alaska had an obligate diapause at a late stage of embryological development. Those from Idaho had a facultative diapause which, if not averted by cold treatment, occurred at a late stage of development. An individual-based model was used to simulate evolution of these traits over a range of season-lengths. Facultative diapause enabled bivoltinism to be a viable strategy in shorter seasons than when diapause was obligate. At transitions from semivoltine to univoltine, and from univoltine to bivoltine, life cycles, populations with obligate diapause adopted a strategy of no diapause (via maternal effects) to avoid the possibility of not hatching until the second year. Results show that simultaneous consideration of complementary diapause traitsy may generate new insights into adaptations to seasonal environments. |
