Malathion is bad. I don't know about using it on sundews, but it totally toasted one of my flytraps and severely damaged my Sarracenia. Pyrethrum (Chrysanthemum extract) worked well on my D. capensis; it killed all the tentacles on the leaves in about twenty minutes, but got rid of the aphids and the plants made a full recovery a few weeks later - the immature leaves that were sprayed managed to open and do just fine as well. Only the older leaves sustained damage. Pyrethrum is also foodsafe and safe indoors (apply it outdoors only,) which is a plus, but is topical, which means that you can miss some of the bugs and they might come back. Acephate (Ortho makes it by the name 'Orthene') is a systemic insecticide, which means it gets into the plant tissues and poisons anything that eats it for a month or so. Acephate is very toxic and for outdoor use only, but it is safe for all CPs so far as I've been told. It's the active ingredient in Ortho Systemic Insect Killer.
If you've caught the infestation early, submersion should do just fine. If things get out of hand, try the Pyrethrum. If you have space outside or in a greenhouse, though, you might as well go with the Acephate/Orthene, 'cuz it's dirt cheap.
~Joe