The effectiveness of any spray for lace bugs depends a great deal on timing except in the case of systemic insecticides. Oil based sprays are very safe and can be effective against lace bugs when the pests are in the nymphal stage. They are much less effective against the adults Your county extension service can provide you with best timing in your area.

Adult lace bugs are less than 1/8" in size and are very hard to see. What we usually see on the underside of rhododendron leaves is the shiny residue of lace bug excrement. What I'm suggesting is that you make sure the damage you've seen is from lace bugs. Thrips and mites produce a similar appearance and would be more difficult to control. Also, unless you're dealing with an absolutely enormous plant, a gallon of spray for one rhododendron is really excessive.

The best control for lace bugs is to apply a systemic drench at the proper time. There are many issues with this approach, so become informed about them before deciding to use one.

I've had the best experience with lacebug control when using a horticultural oil of some kind.

Neem IS a horticultural oil!! But I do agree that proper timing is an issue and will provide the best results.

You should get some immediate effect just by hosing down the plant (make sure you get the undersides of the foliage) with a strong stream of water. Then you can address the lacebug nymphs with the hort oil/neem at the appropriate time. Also, plants grown out of direct sun and receiving adequate water during our dry summers are less inclined to lacebug infestations.

Of course neem is one of the horticultural oils. I meant as opposed to a systemic pesticide.

Lace bugs overwinter as eggs on the underside of the evergreen foliage and in the crannies of the bark. Winter application of your oil sprays, neem or any other, can be extremely effective in interrupting the cycle.

As akamainegrower suggests, less is more! What is important when applying an oil for lace bug is that the spray must be directed to the underside of the leaves.....or don't bother.

In the early spring, check the underside of the leaves for live insects and direct your water sprays at that time.

Good news is that with little effort and no strong pesticides, lace bugs can be managed pretty easily.

Gardengal mentioned an extremely important factor.....lace bugs will prey on plants under stress.