The water shouldn't be very soapy -- a quarter-teaspoon to a gallon or two is more than enough. As long as it's very weak, you shouldn't have to rinse it off.

Severe infestations are often a nutritional problem. Weak plants give off odors that attract insects. If these are annuals, mix up some fish emulsion, worm tea, or compost tea and feed the plants and see if there is some improvement.

It generally isn't advised to feed perennials after the middle of August, as it promotes new growth that isn't 'seasoned' well, and can cause winter damage.

Sue

If they're not indoor plants you can always go after them with just a simple blast of water from a water hose. You'll have to do it a few times over the course of a week or more, but it's effective. Make sure to get under the leaves...

You don't want to do it very late in the day or on excessively overcast+humid days so the plant leaves don't sit around being wet for many hours. That invites a little more disease pressure.

You don't want to do it very late in the day or on excessively overcast+humid days so the plant leaves don't sit around being wet for many hours. That invites a little more disease pressure.

But alternately, you don't want to spray in the middle of the day when the sun is shining, or you'll have sunburn spots on your leaves (this can also happen if there's sun while raindrops are still one the leaves).

Soapy water will kill some plants. It is not recommended for use by the University of Florida Agricultural Dept.