Susceptibility of Diaphorina citri to Irradiation with UV-A and UV-B and the Applicability of the Bunsen-Roscoe Reciprocity Law

Simple Summary: The Asian citrus psyllid is the most widely distributed vector of the Asian form of huanglongbing, and the psyllid is a relatively minor pest in the absence of the pathogen. Pesticides used to suppress psyllid infestations can slow the rate of spread of the disease but not prevent it. Psyllid populations are known to be affected by a number of natural factors, including relative humidity, rainfall, ambient temperature, predators, parasitoids, and entomopathogens. However, in a recent study in Bhutan there was little evidence of these factors being responsible for the psyllid rarely occurring at elevations above 1200 m. It was hypothesised that ultraviolet radiation could be the cause. In this study, we observed how UV-A and UV-B affected the different stages of the psyllid. There was a negative impact of UV-A on all stages of psyllids, but UV-B was the more damaging. At the doses given, when adults were treated with UV radiation, there was little effect on the eggs or nymphs produced. Over the range of doses given, the Bunsen-Roscoe law of reciprocity held true for eggs and early instar nymphs. The result of our experiment suggests that ultraviolet light could be a factor affecting the distribution of D. citri populations. Populations of Diaphorina citri decline with elevation and, in a study in Bhutan, were rarely found above 1200 m ASL. The impact of ultraviolet (UV) radiation, particularly UV-B, on immature stages of the psyllid was proposed as limiting factor. As no studies have been undertaken on the influences of UV radiation on the development of D. citri, we examined the effects of UV-A and UV-B on different stadia of the psyllid. In addition, compliance with the Bunsen-Roscoe reciprocity law was examined. Irradiation with UV-A marginally reduced egg hatch and the survival times of emerging nymphs. Early instar nymphs were little affected by this waveband, but the survival of adults was reduced at the higher doses used. With UV-B, egg hatch and the survival times of early and late instar nymphs declined in proportion to UV-B dose. A dose of 57.6 kJ m(-2) d(-1) reduced the survival time of only adult females. Female fecundity was reduced at high UV-A and UV-B doses but increased at low doses. The Bunsen-Roscoe law held true for eggs and early instar nymphs for different durations and irradiances of UV-B. Eggs and nymphs had ED50 values for UV-B lower than the daily fluxes of this wavelength experienced worldwide. Thus, UV-B could be a factor causing the psyllid to be scarce at high elevations.