More about denying roaches the water they need: Of course you should never keep wet, dirty dishes in your kitchen sink overnight or for any length of time when you’re not in the kitchen. Think about what happens if you just rinse dirty dishes and leave them in the sink: from the roaches’ perspective a sink full of dirty dishes with water on them is an enormous all-you-can-eat-and-drink buffet. You must clean your dishes, dry them, and put them away before roaches can get to them. And then you must clean the sink to make sure there is no food residue remaining where roaches can get at it. So after you wash the dishes, wash the sink and rinse it well. But is that enough?
It might be possible for roaches to get at the water in the trap (the U-shaped bend in the pipe) under the sink. They’re always looking for water and they like to be in damp, dark, confined spaces. There are a couple things you can do to prevent roaches from getting at the water that’s always in your sink’s trap.
First, you can get a sink strainer and make sure it’s in place whenever you’re not using the sink. A strainer should prevent all but the smallest of roaches from going down into the drainpipe. (However, make sure it’s not stopping the flow of water down the drain. If even a spoonful of water collects in the strainer, then you’ve made life easier for the roaches.)
Second, you can spray your sink and drain with a chemical cleaner. Roaches are pretty good at avoiding chemicals that are lethal to them. I recommend putting ammonia into a spray bottle (make sure to label it!) and giving the kitchen sink a good spray every night before you go to bed. Spray an amount equal to a spoonful or two directly onto the sink strainer and down the drain. Roaches will smell the ammonia and will avoid the sink entirely for several hours. You might find a dead roach that has fallen into the sink and come in contact with the ammonia. When I see this, I smile. Plain household ammonia is quite economical when you buy it in a large jug. [You can also use household bleach the same way. NOTE: Be careful you do not to mix ammonia and bleach. Doing so creates dangerous chemical gases that can be harmful to humans, like you! You should also always be careful not to mix any household cleaning product that even contains ammonia with another product that contains bleach.]
I am not certain if roaches actually go on water-seeking expeditions down the kitchen sink drain and into the pipe to the water in the trap, but it seems possible and if you want to get rid of roaches you must take away their access to water.
Rid of roaches 