Tracking the early spatio-temporal dynamics of phytoplasma multiplication within its leafhopper vector

Transmission of phytoplasmas, recalcitrant, yet-to-be-cultivated bacteria, by insect vectors depends on acquisition and subsequent pathogen multiplication within the insect body. However, the influence of acquisition duration on early colonization dynamics remains poorly understood. This study clarifies the spatio-temporal patterns of Flavescence dorée phytoplasma (16SrV-D) multiplication in its leafhopper vector Euscelidius variegatus during the early stages of infection when acquisition access period (AAP) is short. Insects were exposed to two acquisition conditions: a short 4-h AAP, simulating incidental feeding, and a 2-day AAP, considered the minimum threshold for effective acquisition. Phytoplasma load was quantified in the head and body over time, and these data were integrated with published data covering longer AAPs (7-14 days) and post-acquisition periods (up to 42 days). A Bayesian hurdle-lognormal model was used to describe temporal dynamics and estimate pathogen multiplication rates across studies. Very short AAPs led to significantly lower phytoplasma loads after an 8-day latent period compared to 2-day AAPs, highlighting the importance of feeding duration for efficient colonization. Model predictions indicated that phytoplasma load after short AAPs increases gradually and follows similar temporal trends to those observed for longer AAPs, but remained consistently lower across the 40-day post-acquisition period. Nevertheless, empirical and model-based estimates suggest that even brief feeding events, particularly 2 days, can yield pathogen loads approaching the minimum transmission threshold (10³-10⁴ genome units ng⁻¹ insect DNA) in some individuals, as suggested by upper CI bounds, with potential to sustain infection and facilitate transmission. These findings shed light on how acquisition duration shapes early phytoplasma dynamics in vectors and offer insights into transmission risk under natural conditions where incidental feeding events may occur.