By 'balloon-like' do you mean orb shaped? (Rather than flat.) What color is it?

black, orb shaped

I'm afraid that nothing comes to my mind with that description. Does this critter have 6 legs, does it fly, does it munch on the leaves (leaving holes or tatters) or does it suck the plant juices?

I'm needing more information so that I can get a picture in my mind.

its a real mystery, I can't find it in any bug pictures either, but I sure have a lot of them! No wings, pretty big critters. Six legs. Totally defoliates the plant. Actually I can't think of anything to compare them to...except maybe grasshoppers, but they don't hop, just walk.

I can't help wondering if the bug you're describing (which almost sounds like a blister beetle to me) is just an innocent bystander and the real culprits are tomato worms, often unseen because they blend in so well with the leaves, and they also leave lots of poop.

In any case, I hope you will be able to take a picture of your mystery bugs this season. Like the old saying goes, a picture is worth a thousand words, and it goes a long way in insect identification!

I called it a blister beetle, until I looked at web pictures and they show a totally different little striped bug. This has a body very balloon like-(similar to a black widow except not round and shiny of course). The blister beetles I see in web pictures have shells with stripes. I guess we'll have to wait for a summer attack. But thank you all for your time.

There are dozens of different species of blister beetles, with a variety of body shapes, wing sizes and color patterns. There are black ones with very round bodies. See the link below. Yours might not be exactly like these, but they might be "cousins".

Here is a link that might be useful: Blister Beetles family Meloidae on BugGuide