The ever-popular ‘Annabelle’ hydrangea (Hydrangea arborescens ‘Annabelle’), with its big pompoms of white flowers is often attacked by the hydrangea leaf-tier (Exartema ferriferanum), as are other white-flowered smooth hydrangeas (H. arborescens). It’s a small brown moth whose larva has the annoying habit of gluing hydrangea leaves together at the stem tip. Thus sheltered from any predator, the small green, black-headed caterpillar can munch away to its heart’s content on the tender young leaves and flower buds trapped inside its shelter.
Not only is this unsightly, but it often prevents that stem for flowering at all that year… and you didn’t grow ‘Annabelle’ for her foliage!
After about ten days, the caterpillar stops eating and morphs into a chrysalis. About a week later, the adult moth appears and heads off to spend the summer… who knows where? But then it returns in late fall to lay its eggs. There is only one generation per summer.
What to Do?
To nip the current season’s infestation in the bud, open the leaves manually, then hand pick the caterpillar and either crush it or drop it into soapy water. If you do that soon after the leaves are tied together, you’ll save that season’s bloom.
To Prevent Next Year’s Infestation
Saving hydrangea flowers by opening the leaves one by one is not such a chore when it happens just once. It’s a major annoyance, though, when it happens annually. And once the leaf-tier is in your neighborhood, it does tend to come back year after year. So you’ll need a different strategy to control this pest.
It’s important to understand that the moth lays her eggs on hydrangea stems in the late fall, usually only one per stem. The eggs overwinter there, then, in early spring, as the shrub buds out, the young caterpillars crawl up the stem and invade its upper leaves. And that gives you a way out.
Early in the spring, cut your smooth hydrangeas (remember this insect attacks only that species: more on that below) back as close to the ground as you can. Since this species flowers on new wood, this will not affect its blooming. But if you pick up and compost the cut stems, you’ll have eliminated the leaf-tier’s eggs, thus preventing the infestation.
You see, the female moth tends to lay her eggs low down on the stem, sometimes so low you can’t cut back that far. Still, if you prune the plant back to about 4 inches (10 cm), you ought to get most of them. You can then open the tied leaves of the few insects that do survive as soon you notice them.
Another possibility is to spray the stems with Btk (Bacillus thuringiensis kurstaki), a biological insecticide, as soon as they show the first signs of growth. That will kill the caterpillar as soon as it emerges from the egg.
Not All Hydrangeas
Note that the leaf-tier attacks only one species of hydrangea: smooth hydrangea (H. arborescens). There is no need to prune, spray or otherwise treat any other species.
Also, I’ve been surprised and pleased to see that the insect doesn’t seem to be interested in the new pink-flowering smooth hydrangeas, like Invincibelle Spirit II, but seems to stick to the white-flowered ones. Is that just a fluke or are other gardeners experiencing the same thing? Do let me know!