Perhaps you used Bayer 3 in 1 on those new plants? I don't know how long ago that, or a similar solution was used, but you should probably drench all the potting soils with this solution. But before you heed this advice, check the label to see the frequency of application. You need to do all this outside though, in the shade, and allow several hours before bringing those plants back inside.
I don't use any insecticides, or for that matter, any "cides" whatsoever inside my solarium. What I do use are those blue and yellow (mostly yellow) sticky cards. They are completely non-toxic and pesticide-free. You will be amazed how those cards will load up with tiny insects within a short period. I use them by the dozen each year and change them out when the card gets filled with insects (many 100's if not over 1000 insects per card). You can purchase them on Amazon and eBay by the dozen and get the larger ones if available (more bang-for-the-buck).
after 2 wks.
8 wks. - almost ready to replace with a new one