Factors regulating the phototactic behaviour of cigarette beetles
Adult cigarette beetles are known to be attracted by sources of light and also to conceal themselves in dark places. Knowledge of the factors that regulate their positive- and negative-phototactic behaviour is important for estimating the practical value of light traps, but little information has been available until now. In this study, we observed the daily rhythms of the phototactic behaviour and locomotor activity of unmated males and females and oviposition-experienced females in a glass tube under 12L:12D photoperiods to determine factors connected to such behaviour. The observations revealed that adults exhibited higher locomotor activity and positive-phototactic behaviour from close to the end of the light period to the middle of the dark period, irrespective of sex. They then altered their phototaxis and became cryptic towards the end of the dark period and remained quiet until close to the end of the next light period. Males exhibited higher positive-phototactic behaviour than females. However, females became less cryptic and responded to light sources more intensively after oviposition. These results suggested the presence of some bias in the sex ratio and gravidity of light-attracted insects. We therefore examined the sex ratio together with the clutch size of the females among light-trap-captured and hand-collected insects in a chamber in which a heavily infested tobacco case had been placed. It was found that the insects captured by the light trap were male-biased, while the sex ratio of those collected by hand was almost even. The number of matured eggs in the ovaries of light-trap-captured females was slightly fewer than that of hand-collected females. In addition, the catches by the light trap were highest during the early dark period. All these results are coincident with those of the previous glass tube assay.