QUESTION: I have noticed insect infestations in the plum trees this summer. We have an Imperial Epineuse plum tree that is covered with sticky sap and thousands of little white insects. I began to wonder what these insects are, and asked our local insect experts.
ANSWER: We canvassed the local prunus, from our two “Italian prunes” to the neighbor’s wild Italian type to some ornamental prunus on the Seattle U campus last night. One tree was infested with white apple leafhopper which took off in clouds when a branch was disturbed. These critters go thru 5 instars and leave their little white shells on the leaves. Both as larvae and as adults, their body shape is torpedo. Leaves can get that stippled look from their early season feeding. In spite of their name they will infest plums too.
A similar insect is the potato leafhopper which the experts ID by the nymphs being able to move sideways as well as forward and backward! The fruit pickers hate them because their faces are right by the leaves so they get them in their eyes, ears, etc., while picking.
The neighbor’s wild Italian type plum is terribly infested with what we are assuming is the mealy plum aphid because it has a powdery coating. But some leaves higher up and out of reach are getting quite curled so maybe it really is the leaf curl plum aphid. The undersides of the leaves are cheek by jowl aphids and the lower leaves on the tree are very sticky and shiny from honeydew. But when the tree is disturbed there is no cloud of insects. Some of the aphids have developed wings so they’ll be leaving soon. Interesting insects: after feeding on prunus from spring to summer and reproducing rapidly, they grow wings and take off for reeds and cattails up to 30 miles away in late summer and migrate back to fruit trees to lay eggs in the fall.
Guess we’d support the trees by giving them an inch of water a week, maybe running a soaker hose circling the dripline for an hour, until harvest. Spraying is marginally effective and re-infestation occurs quickly.