Leaf curl aphids on ash. A perennial problem but easily treated. Aphids come in a wide variety of sizes, shapes and color and can be found feeding on foliage, twigs and stems, tree trunks and even roots. There are thousands of species of aphid but each species tends to only feed on one particular plant. Controlling aphids depends on a multitude of factors depending on what type of aphid, how many, what the host is and we also have to consider protecting honeybees and other pollinators if choosing to use a pesticide.
Giant conifer aphids typically found on spruce, and sometimes found congregating on sidewalks, patios or foundations.
The aphids in this picture aren't visible but it's easy to see the sticky mess they are making on this rose bush.
Again the aphids on this aspen tree are not visible in this image but it's easy to see the evidence that they're there in large numbers.