Eastern tent caterpillars do not change into Monarch butterflies. The tent caterpillar moth is reddish-brown with two pale stripes running diagonally across each forewing. The mature caterpillars are resistant to danged near all pesticides, including Bacillus thuringiensis var kurstaki, but the young are open to both biological and chemical death. They are protected by the "tent" from rain, etc.

Do what my old grandma did: wrap some rags around a long pole, sprinkle on kerosene, light it up, and burn the buggers out.

(Note: these caterpillars are rarely harmful to trees, just danged unsightly.)