The only thing I can think of is red-humped caterpillars. They'll eat redbud leaves, and they occur in large groups. As the caterpillars grow, you should get a better idea.

Sherry

Thank you MissSherry!

i hope they are not getting any higher up in the tree so I can keep an eye on them. Today was a good day for caterpillars. Found cloudless sulphur and sleepy orange and also this guy, another first for me. At least this was easy to figure out.

If more than 5 pairs of legs, then sawfly.

Sawfly larvae always have three pair of true legs. It's the stumpy prolegs that we need to count. I'm not being nit-picky, lots of people know what prolegs are and it might be confusing. :-)

The picture shows them in a very early instar but take more tomorrow! Remove the leaf on which one is feeding so that you can take the image clearly.

Looking at the picture more closely, it looks like at least two of them have them rear ends stuck up in the air, which is typical of sawflies. I couldn't find any sawflies in my books that use redbud, though.

Sherry

I really need to write this down to remember it better. They don't look as red today.

umnsays sawfly larvae have stripes, uky says spots, ufl says stripes or spots. In Iowa the state has a place that will identify them for you. Idaho labels a completely different insect as sawfly.

ETA wisc says three pairs of true jointed legs, osu says first 3 pairs of legs (and devotes more characters to prolegs.), psu has a decent graphic, European Pine Sawfly — Department of Entomology — Penn State University - Figure 1. Mature European pine sawfly larva. k-state only mentions prolegs, mimastergardener mentions both.

ETA umn says they are Pristophor, osu saysCroesus, and wsu says
There are dozens of sawfly.

Without counting feet, they look like sawflies to me.

Sherry

I am going to try to keep an eye on them. But critters I want to keep an eye on are piling up. Like these

Iris, that looks like the larvae of a sumac flea beetle, which piles its poop on its back as a deterrent against predators. Other species do the same thing, but your pics strongly resemble that flea beetle. What plant it it on?

Sherry, I'm going with your first suggestion of Red Humped caterpillar. Take a look at THIS IMAGE of early instar RH cats.

I've never heard of Sumac flea beetle. Will read up on it later. It is actually on Sumac (the shiny one) it's really cool how inventive these things are.

Inversely, if any abdominal segment is missing a pair of prolegs, then caterpillar.

What does that really mean? Like the looper? I am sorry to ask so many questions. My daughters text books arrived in the mail today. I had to open the box since she is still in Paraguay. Everything from " the study of insects" to "Insect behavior" I was thinking I should read them once she is done with the next Semester, seems I need to start on a much lower level.

One of them was a bit away from the others today. I am seeing 4 stumpy legs. do I add them to the true legs? They all have their behind up in the air.

Looks like protoleg pairs are missing from some abdominal segments, making this a caterpillar. 4 pairs of protolegs is typical of the moth family that includes the red-humped cat.

Note that the absence of these legs can be determined at a very early instar stage without counting anything. Use tweezers or a toothpick to turn the larva over.

Boy, you are good. They are finally changing their appearance. Looks like they might be working on getting a red hump.

Iris, the only true legs on a caterpillar or sawfly larvae are the three tiny pair at the very front, just behind the head. All of the others are prolegs. Are you sure that you want all of these larvae chewing up your redbud? :-0

You've done a good job of taking pictures.

So far they have just eaten about 6 leaves (and there are two groups). I just really want to figure out what is in my garden. And it's fascinating how different they can look within a couple of days. Good or bad, eventually they balance each other out. The only thing I do intentionally harm to are mosquitos (no pesticides, just slapping them when they are sitting on me) and I do squish ticks I get out of myself or my dogs. Otherwise I would have started a war against the broom moth caterpillars. I have about 10 plants of Indigo ( blue, false). Every one has been eaten down to nothing twice this year. 6 of them look like they are coming back. I hope there won't be another round.

One last update, I guess now it's really easy to tell. Amazing how much they have changed.