When dealing with fire ants or wasps, be quick: carefully walk up to the mound, pour quickly, and then back away fast! The insects that are not killed will try to escape the nest. It is best to do this in the late evening when temperatures are still warm and the insects are near the surface, but not very active outside of their nest. Although the hot water usually does not kill all the ants or insects on the first try, you can at least reduce their numbers especially if you can give them repeated dousings.
If you are bothered by slugs then try a little hot water poured onto the slug, this should do the trick. You can also use boiling water
on termites. Hot water also makes a quick and efficient spray to kill soft-bodied insects like aphids, mites, whiteflies, and mealybugs. The water should be between 120 and 150 degrees, and it should only take a second of spraying to do the trick.
Connect a hose to your garage sink or basement outlet for easy spraying, or use a spray bottle. Woody plants where these insects may reside, tolerate hot water better than some ornamentals. However, you should always test on a few leaves before spraying to see if it is alright to spray the whole plant.
- Tips on protecting orchids from mealybugs (sfgate.com)