I knew several years ago when I saw a small, yellowish beetle-type bug fly off my ornamental oregano that it was an insect that was causing the damage that I was seeing on the leaves. The damage is similar to the damage cause by the rose sawfly larva–sucking on the leaves that then causes burning.
But in this case the damage is so uniform that it almost looks like a fungal disease. Many folks are fooled by this and wind up needlessly applying fungicides. When they go back to their garden centers to report that the fungicides aren’t working, they are told “well, you can’t cure fungus, you can only stop new fungus.”
That’s bunk. You can’t cure fungus if what you’re trying to cure is insect damage. The damage here is caused by the four-lined plant bug. And if the University of Massachusetts will excuse my wholesale copying and pasting of its description of the damage this critter does for a moment, you can read it for yourself:
“Four-lined plant bug nymphs (orange-red with black coloration) are active. The nymphs, as well as the adults, (gold-green, with 4 longitudinal black stripes) feed on the foliage of many perennials. Feeding by their piercing-sucking mouth parts results in numerous, small, round, brown-black spots on the foliage; often mistaken for a disease.” Landscape Message, June 10. You can read the entire Message here.
So, mystery solved: it is an insect, the four-lined plant bug, a very elusive critter, that causes this damage. Since I first saw that one years ago, I have never seen another. But they are quite destructive, I assure you! You saw that for yourself on Monday!