“We need to think of the best way to get you out of here. Can’t be done by night, you heard what happens if anyone moves outdoors during darkness: Caterwauling Charm’s set off, they’ll be onto you like bowtruckles on doxy eggs" (DH28).
Phrase used to describe how rapidly someone or something will react and attack.
Aberforth Dumbledore uses this phrase to describe how quickly the Death Eaters would catch Harry and the others if they wander out into Hogsmeade during nighttime curfew.
A number of similes of this type are common in English, including "like white on rice" and "like flies/bees to honey." This is a Wizarding world equivalent based on how much bowtruckles love to eat doxie eggs.