Is developmental rate polymorphism constant? Influence of temperature on the occurrence and constancy of slow and fast development in Zygogramma bicolorata Pallister (Coleoptera: Chrysomelidae).

Temperature plays an important role in different biological activities of organisms. The relationship between temperature and insect development has long been recognized as an important environmental parameter in modeling insect population dynamics. Although few studies have investigated the existence of developmental rate polymorphism within a cohort, the role of abiotic and biotic factors on such developmental variation has so far been meagerly investigated. The present study was designed to investigate the effect of thermal extremes on the developmental rate polymorphism and its influence on reproductive potential of Parthenium beetle, Zygogramma bicolorata Pallister (Coleoptera: Chrysomelidae). The study will also be helpful in solving hitherto whether the existence of slow and fast developers within a cohort has a purely genetic basis or this developmental polymorphism is presided by environmental factors. Our result reveals a clear bimodal pattern of distribution with two peaks at each temperature (20, 25, 27, 30 and 35 °C) where the first peak represents the fast developers and second peak represents the slow developers. Both developmental variants took the longest duration for development at 20 °C followed by 25, 27, 30 °C and minimum at 35 °C. More fast developers were found at higher temperatures. Slow developing individuals were heavier than the fast developing individuals regardless of rearing temperature. Slow developers have higher reproductive success in terms of fecundity and egg viability than the fast developers. The results of this study denote the constancy of the developmental rate polymorphism within a cohort and the possibility that this polymorphism was owing to the exogenous cues inclined differential rates of mortality.