Mealybugs and scale

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I'm not really sure where to post this, so I apologize in advance if it needs to be moved.

I'm working with probably 100 specimens, give or take, of Vachellia drepanolobium, and they have been developing quite poorly, for the most part: only a few, small domatia and slow growth overall. Part of this is almost certainly that they're mostly unfertilized--I'm sure ants are a huge source of nitrogen, and the two that were fertilized did and are doing much better (they flowered once, and have many more, larger domatia). However, they're also quite susceptible to mealybugs, which seem to go within days from being invisible to covering half the plant. I go up and spray the plants down two or three times a week (I'm only working there part time), and I was wondering if there is a better solution. The person who actually needs the plants for research has suggested that one day we should go up and, together, cover them all with neemoil, but I suspect one application will have little effect. We can't use synthetic pesticides, because these plants will (hopefully) eventually have ant colonies on them which could be killed by the residue, and we can't use biological control because it might interfere with the ant experiments down the road. Spraying down with water seems to only have a very temporary effect. Should I suggest that we repeatedly apply neemoil, and if so how long would it need to be applied for? If not, what are some other solutions you have used that have worked in the past? It seems that ant plants spend so much energy on their domatia and various sources of food for the ants (extrafloral nectaries in this case) that they have little left over to spend on distasteful chemicals to deter insects (though having said that, some of the other acacia in the greenhouse, that aren't naturally any associated, have been very hard-but by both scale and mealybugs).

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If it's helpful, this is a lab in Massachusetts, in the United States. Insecticidal soap has been effective against mealybugs in the short term, but I'm not sure it will have many long term effects and it doesn't work against scale.