Aphids. My plum trees are covered with ladybug larvae each year, then they pupate all over including on individual blades of the grass under the tree, and if I don’t move it in time, the hammock stand. Have you noticed ladybugs or clusters of yellow eggs(ladybug eggs) Garden Patrol might be already at work.
Backyard birds like Finches, Wrens, and Chickadees also swarm the trees and eat the aphids... and possibly ladybugs as well. This is the only time that Warblers also visit my garden.
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