These conspicuous red galls, like a short string of small red jelly beans in the leaf, are caused by a small sawfly. They look bad but are really quite harmless.
Female sawflies are approximately 4 mm long and are shiny black. After emerging and mating in the spring, the female inserts the eggs into the leaf tissue, where the galls and larvae begin to develop.
Later larvae leave the galls, fall to the ground and pupate. There are two generations per year.
Willow is this sawfly's only host.