Try and do all these while the sun is not very strong or in the evening, so the soapy solution won't evaporate too quickly. Plus caterpillars seem to be more active at night time so they'll feed on the "fresh" food you just applied to the foliage of your box trees.

Washing the box trees

When you first spot caterpillars, try and remove as many as you can with your hand. Wear gloves if the thought of touching them scares the bejeezus out of you. You will need to find a way to kill them though, as disgusting as it may sound. I used my garden shoes and took great pleasure into squashing these nasty creatures with it! Pay very close attention and use your hands to reach inside the box tree and at the bottom because the caterpillars are hard to spot as mentioned above. You may need to to a second round once you finish checking your plants.

Grab the garden hose and with the pressure washer on, give the box trees a really thorough wash, again paying close attention, particularly to the webbing areas where caterpillars, larvae, eggs or cocoons hide. The cocoons tend to be surrounded with lots of dried leaves so the areas are not hard to spot, especially if the plant is severely affected. Wash the plant starting from the top towards the bottom, opening the leaves and pushing the stems out of the way with your hands. I found many eggs laying inside the new shoots.

The caterpillars are almost impossible to wash with the hose, as they cling to the leaves with a super sticky slime. But you're aiming for washing and drowning the eggs. The next step will take care of the last few caterpillars you might have missed.

Applying the soapy solution

Allow the box trees to dry (chances are once you're done washing, most of them are already dry) then mix dish soap and water (1/2 cup soap to every 33fl.oz water) in a garden sprayer. First add the water to the garden sprayer, then the soap and mix using the sprayer's nozzle to avoid forming bubbles.

Give the box trees a thorough wash with the soapy solution. The caterpillars, the pupae and the eggs you may have missed during the washing process, will dry out pretty soon. But if you have a serious infestation, chances are even after applying this homemade killer, you missed a caterpillar or two and maybe a few eggs which are to continue their life cycle, turning into moths/caterpillars. That's why you need to continue the fight so...

Repeat the steps above in two days.

Preventing infestation

After that, apply the soapy solution once a week and keep a close eye on your box trees to spot any new moths or caterpillars. If you do, depending on the severity, you may need to either increase soapy spraying or do a deep clean as highlighted above. Chances are after a couple of deep cleaning sessions, you got rid of all these nasty insects and their eggs and you only need to apply this box tree caterpillar killer as prevention.