These little tufts of wool (actually a thick flocculent wax) are aphids and are called, you guessed it, woolly aphids. They are on the underside of the leaves sucking out the juice flowing through the leaf and the leaf will start to curl under as a result. The juice that they can’t digest is excreted as a little sugary droplet known as honeydew. Their sugary droplets create a sticky coating on the ground and leaves below them. Chickadees, bushtits, Bullock’s orioles, black-headed grosbeaks love to eat them. Ants are attracted to the honeydew. Squirrels are also in on the scene, either eating the aphids, licking the honey off the leaves or both probably. I looked at a few of the snipped off leaves that they discarded while foraging and they don’t have any aphids left on them.
They seem especially abundant this year and I doubt there is a single ash tree in the arboretum that doesn’t have them. The interesting thing about woolly aphids is that the first ones to hatch out in the spring are wingless females which reproduce without mating (parthenogenesis). This allows them to produce offspring quickly, which is why you can see them throughout the arboretum at the moment.
I read that woolly aphids typically have two host plants. After a couple of generations in the spring, they will produce winged females that will fly off to another host to feed during the summer. I captured a photo of one below that appears bluish. I am not sure what their second host for the summer is here at the arboretum. As fall approaches the winged females will return to the first host, which appears to be the Oregon Ash. (I remember seeing tons of these flying around in the fall. I posted about it in Woolly Aphids - October 2021.) From here they will produce another generation of both males and females that will mate. The female will then lay her eggs in the bark to over winter.
