Several years ago Jacqui Knight, of the New Zealand Monarch Trust, shared this method of feeding pumpkin, etc. I have tried the pumpkin on fifth instars (see photo below). I peeled the pumpkin, cubed it, put a hole in the middle, and then placed the cubes of pumpkin on the stems of milkweed. If used on first to early fourth, the caterpillars will not thrive.
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Here is a link that might be useful: Search on New Zealand Monarch Trust for Pumpkin

I wish I had that problem! I'm not seeing any Monarchs at all, much less caterpillars! Now that the wild milkweed are more easily spotted (some are blooming), I checked some roadside milkweed plants today....nothing found at all. I even drove by the dam area where I was seeing Monarchs last fall and saw none. However, I did see a Queen in my yard the other day, first I've seen this year!

I have what I call butterfly buddies who share stock with me and I share with them in Virginia

Have you tried emailing any of the people that post sightings on Journey North for eggs and caterpillars. I usually follow the migration on their maps and sighting. The sightings website is a drop down list so you have to select the category. If you click on a sighting in the list, there is an email option.

Here is a link that might be useful: Journey North Sightings and Maps

Back in 2007, sandwhy mentioned that Lincoln Brower raised monarchs on cabbage back in the 60s. Also said pumpkin would work.

http://forums.gardenweb.com/forums/load/butterfly/msg071334123708.html

The thread I've linked from 2009 is the first time I remember pumpkin getting mentioned here.

Here is a link that might be useful: Do your monarch cats eat pumpkin???