Euonymous is an ornamental plant commonly planted in the Midwest. The “Burning Bush” varieties are especially popular for their brilliant red foliage in Autumn. Euonymous plants have a few insect pests. One of them is the Euonymous Scale, a tiny insect that feeds on leaves and stems. The Euonymous Scale creates chlorotic, yellow, halos in the leaves. Large populations can cover leaves and stems.What causes the halo? That was a question I tackled several years ago. Scale insects are tiny, less than 2 millimeters in length. Their straw-like sucking mouthparts, which are up to 4 times the body length can stretch for several millimeters. I used a microtome to cut histological sections through the scale and the plant it was feeding on. I stained the tissue with dyes that highlighted the position and progress of its feeding. I got one excellent thick section that clearly shows the scale mouthparts and its feeding sites in the leaf.
In the image below, the Euonymous leaf is on top and the scale underneath. The thin gold line connecting the scale to the leaf are the scale’s sucking mouthparts. The mouthparts enter the leaf and create dark red feeding sites. The feeding site shows that the mouthparts entered the leaf from the bottom and then branch several times. The branches result when the scale insect drills in one direction, then retracts its mouthparts and drills in another direction. Other images show the mouthparts, which are smaller than a single plant cell, penetrating the parenchyma cells near the upper surface of the leaf. The scale injects saliva into a cell. The saliva digests the contents of the cell creating a nutritious liquid slurry that the insect pumps into its digestive system. The feeding destroys the chloroplasts that give the leaf its green color and the insect pumps the contents chlorophyll and all. This leaves a damaged cell with no chlorophyll.Scale insects settle in a location, then use their long mouthparts to feed from cells in all directions. The feeding damages cells and removes chlorophyll in a circular pattern leaving “Yellow Halos” in the green leaf.