Do you know what kind of tree it was on? Eggs are usually laid on the correct host plant.

Looks like larvae of a potters wasp. Cool! Their little 'pots' come a a wide range of colors and shapes and configurations. The adult wasp will deposit one egg per little pot. Usually, paralyzed caterpillars are stuffed inside for the larvae to feed on while in development.

Here is a link that might be useful: Check this out.

Thank you all!
Thanks for the link rhizo!..Interesting!
It was on a apple seedling.