Journal of Citrus Pathology

Huanglongbing (HLB) is considered the most devastating disease of citrus. The bacterium and vector associated with HLB in Florida are Candidatus Liberibacter asiaticus and Diaphorina citri (Asian citrus psyllid), respectively. D. citri is positively phototropic, and higher populations have been found along edges of orchards exposed to the sun. A survey was designed to determine if D. citri adult populations along edges of orchards varied according to time-of-day and time-of-year in relation to the azimuth of the sun. The survey was conducted twice. Citrus orchards, each divided into 9 sampling areas, were surveyed for D. citri via stem-tap sampling. Orchards were sampled 3 times per day (near sunrise, solar noon, and sunset) and 4 times per year (near the summer solstice, autumnal equinox, winter solstice, and vernal equinox). Time-of-year and sampling area significantly affected psyllid counts (P = 0.0518 and 0.0630, respectively). D. citri adults were most prevalent during the summer solstice sampling period. No overall significant time-of-day effect was observed (P > 0.6). Localization of adult D. citri in sampled citrus orchards did not significantly change in relation to time-of-year (P = 0.0907). Linear mixed regression was used to fit a quadratic equation to log D. citri abundance data in relation to elevation-corrected azimuth at the time of sampling; the fitted model was significant and predicted log D. citri abundance to exhibit a concave-up pattern with increasing elevation-corrected azimuth. This relationship represented in a new form how population counts of D. citri adults in Florida were greatest during the summer.