Photoperiodic time measurement in insects can be affected by interrupting the dark phase of the light dark cycle with short supplementary light pulses. All the photoperiodic responses tested are highly sensitive to night interruption, and the long night effect is reversed by a light break. Most past studies revealed two points of apparent light sensitivity (the so-called A and B peaks), but the response pattern varied considerably among different species. Some species had only one peak (or trough), either A or B. Even the same species may have different patterns of response depending on the experimental conditions. However, it is still not clear whether differences exist in the effect of night-interruption on diapause-averting among different geographic populations. The cotton bollworm, Helicoverpa armigera (Hübner) is one of the most serious crop pests in Asia, Australia, Africa and Europe. The species is distributed across much of China and attacks many host plants. The widely distributed H. armigera encounters a great diversity in climatic conditions in different places and shows different critical day lengths or night lengths (the day length that elicits 50% diapause response) among different geographic populations. In the present study, the photoperiodic response and effects of night-interruption on diapause-averting, in which the scotophases of L9:D15 (Light 9 h:Dark 15 h) and L12∶D12 were interrupted by 1 h light pulse, were investigated at 22℃ and 25℃ in Taian (TA, 36.15°N, 116.59°E) and Kazuo (KZ, 41.34°N, 120.27°E) populations of the cotton bollworm H. armigera. The photoperiodic response curves showed that the incidences of diapause in the scotophases of 11-14 h were significantly lower in the Taian population than in the Kazuo population. The critical night length in the Taian population at 25℃ and 22℃ was 11 h 42 min and 11 h 30 min, respectively, and 1h 20 min longer than that in Kazuo population (10 h 30 min and 10 h 18min). In all night-interruption experiments, the incidences of diapause in almost all positions of light pulse in the Taian population were significantly lower than that in the Kazuo population. However, the diapause response curves were similar in the two populations. Under L9: D15, the incidence of diapause was inhibited most effectively in both the Taian population and the Kazuo populations when the light pulses were placed at 9-11 h in the darkness. Under L12:D12 the incidence of diapause was lowest at 25℃ in both Taian population and Kazuo population when the light pulses were placed at 3-4 h and 10 h in the darkness. However, the Kazuo population only showed a peak of diapaus-averting occurring in 3-4 h in the darkness at 22℃.
Our results reveal that effect of night-interruption on diapause-averting is different among different geographic population. The southern Taian population is more sensitive to night-interruption than the northern Kazuo populations, but the most highly photosensitive position is similar for both populations.
A light pulse plays two possible roles in photoperiodic time measurement. First it ‘initiates’ or ‘resets’ the clock; this has been called the ‘entraining agent’ or ‘priming agent’. Second it ‘measures’ a critical duration of day or night; this has been called the ‘inductive agent’ or ‘terminating agent’. The present study in H. armigera reveals that a light pulse can reset the process measuring night length, and thus influences the incidence of diapause.