Investigation on rhythmicity in oviposition of an aphidophagous ladybird, Cheilomenes sexmaculata (Coleoptera: Coccinellidae)

Abstract

The rhythmicity in insects has been widely studied by researchers showing changes in the physiology, behavior, and population synchrony of the insects. One of the most important rhythms which control the population is oviposition rhythm. Oviposition/egg-laying rhythm in an aphidophagous ladybird, Cheilomenes sexmaculata (Fabricius), was found to have two components, viz. complete ovipositional cycle and diel cycle. The complete ovipositional cycle was perceived at three levels, viz. a polynomial curve, two lunar rhythms (ca. 28 days), and daily oscillations. Lunar rhythms are deciphered for the first time in the ovipositional cycle of a ladybird. In a diel cycle, most of the oviposition (71%) occurred during scotophase, with a peak (48%) during early scotophase (i.e. 1200–1500 h). Occurrence of most of the oviposition during a phase when predatory activities are low (i.e. scotophase) appears to be a survival strategy. A mathematical model was developed to predict the ovipositional cycle. Fourier analysis has been applied to the data to find out the periods of different cycles of oviposition rhythm with the help of Matlab software. The average oviposition duration was 63.3 ± 0.4 days, with average lifetime fecundity of 975 ± 247 eggs. Two lunar rhythms with a periodicity of approximately 28 days were found within a complete ovipositional cycle. Periodogram obtained from Fourier analysis suggests that a diel cycle of 3.83 slots, i.e. after every 3.83 slots of the day the number of eggs reduces to minimum.