dodder (plant)
莵 refers specifically to the dodder plant, a parasitic vine. Its meaning is narrow and botanical, without broader abstract extensions.
莵 is a phono-semantic compound with 艹 (plant) and a phonetic element related to 兎 (rabbit). The exact historical development of the lower component is uncertain, but it likely contributed the sound while the grass radical indicates a plant.
The grass crown 艹 sits above a shape that resembles a rabbit (兎) with an extra dot. Picture a rabbit hiding under a tangled parasitic vine: that vine is dodder.
For ト, imagine a dodder vine wrapping around a toe: toe -> ト, and the plant clings like a parasite.