lbbs said:

I have a bad case of aphids on my plum tree. Would it be bad for the tree
if I spayed oil on the tree to kill them and then a day late, spray the tree
with a water gun to wash off the oil. I tried insecticide soap, but did
not help much.

It will be a bit risky even if you use an oil specifically formulated for spraying
during the growing season, sprayed in the evening of a not-so-hot day. I'd wash
it off the next morning, not wait a whole day. The oil only works on insects it
covers, and has no residual effect.

Better you should use an oil spray next year early in the season, and figure out
why your tree is so attractive to pests (overdosed with nitrogen fertilizer is one
possibility, water stress another).

Regular blasts of water, followed by foliar sprays of seaweed might be a safer
control.
--
Pat in Plymouth MI

Any technology distinguishable from magic is insufficiently advanced.
(attributed to Don Marti)