plant louse, aphids
蚜 refers to small sap-sucking insects that infest plants: plant lice or aphids.
蚜 combines 虫 (insect) with 牙, which likely contributes the sound. The exact historical development is uncertain.
An insect 虫 with tusk-like 牙 mouthparts piercing a plant stem: that is an aphid.
For カ, imagine a car covered in aphids after parking under a tree: car -> カ, and the insects swarm the vehicle.