Acarapis woodi feeds inside the tracheal tube just behind a bee's first thoracic spiracle. Mite load scars and narrows the tube, so the effective airflow to the flight muscles falls roughly linearly with load (capped near 96% blockage — a trachea is never fully sealed):
- Blockage = clamp(effLoad / 13 × 100%, 0, 96%), where effLoad is mite load per trachea, halved by hygienic/resistant stock.
- Airflow = max(6%, 100% − blockage × 0.85).
- Population growth rate r = seasonFactor × resistanceFactor × (0.4 + infestation/100). Winter clustering (broodless, tightly packed bees) accelerates spread the most; summer foraging dilutes it the most.
- The mite-population readout integrates dP/dt = P·(r − 0.75) each simulated second, so it genuinely grows or decays over time rather than only reporting a static label — watch it climb in a resistant-free winter colony and fall once you enable resistant stock.