Sounds like an opportunity to check on what rinsing can do. You could spray some other leaves with the same stuff. Perform different levels of rinsing (rubbing each leaf under running water vs. just running water over the leaves) and then feed some to the guinea pigs. That way you could find out what you need to be doing with regards to rinsing. You might also find out that rinsing would not have helped.

If you are going to raise that many cecropias, I suggest either a lot more containers or much bigger containers.
I used to raise cecropias in high numbers in close quarters. Worked fine for years. Then one got sick and then in short order more than half were dead. The mechanism cats use to rid themselves of toxins works great when they are in a tree but bodes poorly for their brethren when they are cooped up.

yeah I think I am just going to avoid collecting from that location until I better figure out what they sprayed.

I lost around 50 of them and there are still few more that might go =(. I just sorted them as quickly as I could to separate out the dying cats and get rid of all the poisoned leaves before any more ate them. Then I washed all their containers before splitting them back up again. There are still over 100 and they are split between 8 cool whip/food containers. I moved the biggest ones out last night to a large 10 gallon storage bin, but most all of those were poisoned. I know I have too many. Most I will be releasing or giving away soon. I only intend to keep 20 or so to the 5th instar. I have some luna cats I want to keep too.

You said you had birch and box elder in your yard. You should be using those instead of going elsewhere for food. Plant another box elder. In three years, it will be another ample source.

I had those in my parents yard, those also were sprayed though. But as I mentioned above I collect leaves from around my apartment. I have no yard currently. I also use wild cherry, never tried box elder and very few birches are around here.