Joan, are they potted individually yet? If so, you can spread your fingers over the pot tops/dirt, plunge them gently upside down in a container of water that you've add a few drops or a squirt of dish soap to.... Or, if in seed flats, use your fingers and rub them off like I did my hellebore seedlings yesterday. Or use a paper towel if you wish - then check each day to make sure no new hatchlings are there should you have missed any. Darned aphids in this mild climate. I bought insecticidal soap today but didn't find any critters left to spray. Yet. The lady bugs are great, but it's a little early to find them locally here and by the time they been ordered and have arrived....well, you know what aphids can do to tender new growth.

Imidacloprid, the most common applied nicotinoid, is effective against many insects and even fleas on pets. It can be added to irrigation water, though it is somewhat less toxic to mammals and birds than other insecticides like carbamates or organophosphates. Neonicotinoids are used on about 95 percent of corn and canola crops in the US, the majority of cotton, sorghum, and sugar beets and about half of all soybeans. Also, they have been used on the vast majority of fruit and vegetables, including apples, cherries, peaches, oranges, berries, leafy greens, tomatoes, and potatoes, as well as cereal grains, rice, nuts, and wine grapes. Imidacloprid is possibly the most widely used insecticide worldwide. In addition to outright toxicity, sub-lethal doses of neonicotinoids may interfere with the bees’ ability to forage, learn and remember navigation routes to food and back to the hive, which in itself is a death sentence.

ants milk aphids.... for the honeydew ... they do not cause the aphids.. nor do they spread the aphids .. get rid of the aphids ... and the ants will mind their own business ..... there is little point in trying to control ants... and i am NOT talking about the down south man eating ants ... lol .. now.. if they come in my house.. its all out warfare ... or undermining the cracks in my asphalt .... ken

Insecticidal soap may work okay. It is a little early to be spraying them with anything - tender little things. I don't remember having ever sprayed tomato seedlings quite as young as yours. I haven't had good luck with aphids and rotenone/pyrethrum. Insecticidal soap doesn't work very well in the greenhouse, from my experience. Since the bugs have to dry out to die . . . maybe, the high humidity in the greenhouse lowers the effectiveness. If we can every get a warm, still day in this neck of the woods, putting plants outdoors in the shade and spraying them with the soap may be a good way for you to get past the problem. I have done that often. You probably have quite dry air indoors and taking them outside might not be necessary . No sun! You may also rinse them with a clear water spray after a couple of hours. Now, it would be good to know if anyone has used insecticide on such tiny seedlings. Steve