moth larva, grasshopper
蟖 centers on insect larvae that feed on plants, specifically moth larvae and grasshoppers. Both meanings are specific insect pests, so the core idea is a plant-eating insect in its larval or adult form.
蟖 combines 虫 (insect) with 斯, which likely contributes the sound. The exact historical development is uncertain, but the structure suggests a phono-semantic compound for an insect name.
The insect radical 虫 on the left tells you it's a bug. The right side 斯 looks like a bundle of sticks or grass, so imagine a moth larva or grasshopper hiding among the plants.
For シ, picture a moth larva on a leaf, and think of it as a tiny 'she' (シ) insect. The sound 'she' cues the reading シ.
Eobiana engelhardti (species of bush cricket)