I would hit them with a pyrethrin spray first, followed up by a dose of insecticidal soap the next day. For both, be sure to drench the plants completely. Wait a few days, then repeat. Do isolate the plants from your other semps, if they are in pots. This treatment will knock the bug population way back, after which the beneficial insects should be able to control the problem. This is what I do when the occasional aphid breakout occurs. Only a few semps seem to be susceptible, most apparently are not.
