The overwintered adult beetles are found on the new foliage of  host trees, willow and poplar, in late April and May where they feed and lay clusters of eggs.  The larvae eclose from the eggs in a few days and mature rapidly, producing second generations in June and July.  In September, the adults seek overwintering shelters under the bark of trees or in debris.   
Imported willow leaf beetle Plagiodera versicolora (Chrysomelidae)
A newly molted larva showing the nine eversible glands that occur on each side of the body. The glands secrete a fluid that repels both predators and competitors.
Time lapse video of molting larvae in a feeding cluster.
Time lapse video of a cluster of feeding larvae.
Larvae moving along the midrib of the host plant.
The larvae are skelotinizers feeding on the soft tissue of the leaf and avoiding the tougher veins.
Larvae moving between feeding sites use the midrib of a leaf as a common pathway. See video below.
The light colored larvae are newly molted.  Their exuviae are on the surface of the leaf.