About 1″ to1 1/2″ smooth yellow caterpillar with black stripe down the back.Found in central Maine. Don’t know what it was on. Have been looking at a lot of pics but haven’t found a match. (PA)
Your ‘caterpillar’ is an elm sawfly larva.
The sawflies are actually wasps not flies or butterflies or moths so this is really not a true caterpillar but called a larva.
Bugguide says this about this larva
“The largest North American sawfly with larvae reaching almost two inches. Larvae are a yellowish-white color and possess a black dorsal stripe. While feeding, the larvae usually coil their posterior around a leaf or twig.
At rest the larvae roll into a characteristic tight coil. The larvae spin tough, papery cocoons in the litter or just below the surface of the soil. Pink coloration is not common, most larvae are green to yellow in color.
Larvae have chemical defenses, ejecting fluids from glands near spiracles; often coil hind end around twigs.”
This larva feeds on foliage of elm as expected but also on birch,maple, plum, poplar, apple, willow,and basswood trees. After feeding the larva overwinter in tough papery cocoons then in spring it emerges as the adult wasp.
The adult wasp can chew small twigs to the point of breakage as they are feeding on the sap.
This insect is not considered a major pest but can damage trees or kill them if high infestations occur year after year.