Looks like some kind of aphid. For aphids, I usually take a squirt bottle that can produce a strong stream and blast them off the plant with plain water. They usually can't make it back onto the plant. It's cheap, easy, and effective. Check plants daily to see whether they're coming back, repeat as necessary.

Thank you so much! That's actually exactly what I did yesterday after reading an article about other tomato pests. I couldn't find any info online about this particular insect, which is why I posted in this forum. In your experience do the plants usually bounce back pretty quickly? Also, is there a way to guard against other pests and diseases? Thanks again!

Yes, the plants bounce back, provided the aphids haven't transmitted a disease. Tomatoes are very good at recovering from physical trauma. I would prune off any seriously damaged foliage. New growth will happen quickly.

As for guarding against pests, it depends on your philosophy. Are you someone that doesn't tolerate any plant damage at all? Are you more holistic? Organic? What is the environment the plants are growing in? Do you have an established population of good bugs, i.e., predators that eat pest bugs?

Personally, I think that good cultivation practices like not overfertilizing your plants (plants that have had their growth pushed too far by high-nitrogen fertilizers are more attractive to pests), keeping garden plants away from overgrown, weedy areas that are likely to harbor pests and diseases, and nurturing beneficial predators, are important. But other than that, prevention through chemicals and application of products is just more fuss, time, and toxins put into the garden. I have a good balance of beneficials, so I just stay vigilant and inspect plants daily, especially the undersides of leaves, and take care of any problems if and as they arise. If I see a few aphids here and there, I leave them for the beneficials. If I see them starting to really colonize part of a plant, though, I deal with them. If a problem occurs that can't be taken care of manually by picking off pests ((infestation of flying/jumping pests or pests like caterpillars that can wreak a lot of destruction very quickly), there are many options from relatively nuclear options to various organic options, to turn to, depending on your personal philosophy.

Disease is something else. If you are in an area that reliably gets fungal or bacterial disease issues, preventative spraying of fungicides on a regular schedule is a key tool in keeping your plants healthy longer and slowing down any diseases they do get. As with everything else, there are more hard-core options and more organic options. Some people choose not to spray until they see signs of disease. I personally think that, in any area with appreciable humidity, tomatoes are so prone to so many fungal and bacterial problems that preventative spraying is a good practice unless you are philosophically against it.

Thank you so much for all the great info. We live in Maryland, so the summer temperatures are in the 70s at night and anywhere from 80-100 during the day. We do have very humid days here, so I don't know if that means I should do a fungal or bacterial spray.

I freed the plant of all the insects and I also removed the damaged leaves on Tuesday. It's still not perking up and the lower leaves are now yellowing too :( Maybe it will take more time to recover? My other tomato plant looks great... I'm not sure what's going wrong with this one.

Fyi...I don't really have a philosophy regarding gardening. I guess I'd prefer not to spray with too many chemicals. I actually only have these two plants so I figure it should be pretty manageable. I thought I'd start small and see how I do. Both of the plants are in containers on my deck. So, there aren't any weeds or anything to take care of.

Thanks again for your help.

Some of the damaged leaves will get worse if the damage was heavy, some will get better. Some will stay as they are now. But overall, the plant will soldier on and put out new growth to compensate, and the overall picture and the new growth is more important than the recovery of existing leaves. Just monitor to make sure new growth looks healthy.

If the weather is very hot, you might be seeing normal drooping/wilting/leaf rolling/general unhappiness during the hot hours of the day. This happens with totally healthy plants as well. Check the plant first thing in the morning, after it's had the night to cool down and rest. If it looks OK then, it's the heat that's bothering it. Also, stick a finger down below the surface of the soil twice a day to check moisture levels. Excessive heat adds extra stress, so you don't want the potting mix to be too sludgy or too dry. Water as the need arises (it starts to feel dry an inch or two down), and water deeply until it runs out of the bottom of the pot. If you find yourself needing to water frequently, you will need to feed more frequently as well.