When a tree's sap stagnates, it breeds borer worms.
Tips
history
Not used alone in everyday Chinese; it survives in literary compounds like 蠹虫 (a parasite, a public pest) and the idiom 蠹国害民 (to ruin the state and harm the people). The doubled 虫 at the bottom is its insect radical.
memory
Picture two insects 虫 gnawing inside a stored sack 橐 - that bag-full-of-worms image is exactly a 蠹, a hidden thing that eats you from within.
First of the paired insect radicals 虫 at the foot of the character. Two bugs together intensify the image: a swarm of borers eating wood, books or cloth.
phonetic
橐tuó
sack; bellows-bag
The bulky top is a contracted 橐 (a tied storage sack). It hints at something enclosed being eaten away and loosely steers the dù sound; it is not a productive phonetic elsewhere.
