Optimal diapause strategies of a grasshopper, Melanoplus sanguinipes

Previous analyses of diapause in insects have most often focused on the timing of the switch from non-diapausing to diapausing offspring in bivoltine populations and have assumed that diapause is irreversible or that the insect cannot survive winter if not in diapause. Many insects exhibit more flexibility in their life cycles, such as the age at which diapause begins, and facultative diapause, that may influence the evolution of different diapause strategies in different environments. The grasshopper Melanoplus sanguinipes F. (Orthoptera: Acrididae), has a very wide geographic range over which diapause characteristics vary greatly. Embryonic diapause in this species may be under maternal control, may be obligate or facultative (i.e., may be averted by cold temperature treatment of pre-diapause embryos), and embryos may enter diapause at different ages. Diapause traits were examined in two populations of M. sanguinipes from very different environments. In the population from a temperate climate (Idaho, USA), diapause was facultative, i.e., pre-diapause embryos averted diapause when held at 5° C for 90 days at all ages tested (7 days and older). The Idaho embryos entered diapause in late stage of development if held at 22° C for 30 days or more. In populations from subarctic Alaska, USA, embryos also entered diapause in a late stage of development, but diapause was obligate and could not be averted by chilling in the pre-diapause stages. Simulated evolution of these traits over a wide range of season-lengths showed that late stage diapause is an essential trait in very short season environments, resulting in early hatching, and a semivoltine life-cycle. 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 enable univoltine life cycles.