I’d love this for house flies.
Fruit flies will be attracted to the monster, fly in expecting to land on the water's surface tension but the soap broke the surface tension so the flies end up drowning. It is surprisingly effective.
> An insect going for a drink is in as great danger as a man leaning out over a precipice in search of food. If it once falls into the grip of the surface tension of the water—that is to say, gets wet—it is likely to remain so until it drowns. A few insects, such as water-beetles, contrive to be unwettable; the majority keep well away from their drink by means of a long proboscis.
-- On Being The Right Size (1928) by JBS Haldane
... Though I usually quote it for what it says about a horse falling down a mineshaft.