You can usually blast off aphids with a strong spay from the hose.

Kathi

I use a blast of water , and or soapy dawn water.

Dealing with aphids is not usually a one time activity. The aphids you see today may not be the ones you sprayed yesterday. Knock them off with water and expect to do it more than once.

I guess it all depends on how much water your schefflera can safely take right now. I would wait until it needs a good watering to apply the Bayer systemic if that's what you're going to do, since you'll need to water it in or apply in a pre mix solution. Neem and the systemic work differently, so I don't see a problem with using both approaches as long as the soil isn't overly saturated for too long by the treatments. Neem also makes the plant more susceptible to sun burn, so keep that in mind.

There will be many generations of aphids on your plant from egg-juvenile-adult, as well as below soil level. Unless you completely saturate the plant and the soil with neem, it won't knock them all out with a single application. Even then, I'm not certain that neem will kill unhatched eggs. And it definitely won't prevent new infestations from happening. I'm not a huge fan of systemic pesticides because they also kill beneficial insects (as does neem), but they are necessary at times to save a plant.

A cotton swab dipped in an alcohol solution (or hand sanitizer) will make quick work of those aphids that are visible until you can treat with a systemic. Keep in mind that many systemics come with added fertilizers (designed for shrubs and roses)- which your bonsai may not react well to in its pre-fab concentration. There are a few without fertilizer, I would suggest one of those instead and continue with your own fertilization schedule.

I've never used a systemic on smaller plants, just shrubs and roses and a very large copse of prickly pear- all badly infected with various scale insects. I do plan to apply systemic treatment to a potted crested opuntia (almost impossible to manually clean) with scale soon, but am waiting until it shows a spring break of dormancy first, when a soil drench of systemic pesticide solution will be possible. Until then, I'm manually removing the scale as I can once a week to keep things under control. I'm also keeping it separate from other plants to prevent spread of infection.

I've found that aphids are often symbiotically socialized with ants that 'farm' them for their honeydew and aggressively protect them from predators. The ants will protect the aphids, almost like shepherds with sheep. Spraying off aphids will eliminate some, but I've also witnessed ants tenderly carrying aphids back to the plant from the soil. It's very sweet, but obnoxious.