Q. Can you dispose of a spider or an ant by throwing it into the toilet or sink?

A. These insects usually do not drown or die immediately when thrown into water and often successfully climb the sides of the vessel. However, when flushed, it is unlikely that they may remain alive and survive inside the sewage pipes.
Although it may not be a certainty or a “p’sik reisha” that the insects will die when thrown into a toilet bowl, Horav Shlomo Miller’s Shlit’a opinion is that one should avoid throwing them into the toilet or sink and then having them flushed or washed away, as it may be considered a dovor hamiskaven or something one desires and intents.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a