Larvae of the pine sawfly
up here in MI, IIRC. 'Ours' are OD green in color
and every so many seconds they sway in precise unison, as if following a military command. This spooks predators away and may startle human observers.
Some guys fret when the see 'regiments' of 'em munching on pine needles (esp the many reds), not realizing that there are no three year old pine needles. Branches once laid bare are scarcely recognizable within two seasons as new growth expands a tree's crown and covers the bleak limbs they had cleaned off.
All that said, it could be a
lot different if they attacked forage crops and barely left anything.
This is from experience and memory. Google info may supersede mine.
t o g
Edit: I Googled army worm and read that there is more than one specie so named, others being a moth that loves grasses. The best organic control is said to be BT, a favorite of mine. Carbaryl (Sevin) works but is not the best or cheapest option.