Several readers have sent me pictures of oak trees with dead branch tips, wanting to know the name of this alarming disease and how to treat it. Although oaks suffer a host of foliar diseases, these images are not about the arrival of a new pathogen, but the departure of a primeval insect.
It has been several weeks since the exit of Brood X of the 17-year periodical cicada, and I know I’m not alone in missing these enchanting ruby-eyed creatures and the otherworldly soundtrack of their chirping. But we knew that there would be a price to pay for this spectacle.
After mating in May and June, the cicadas pierced the living layers of pencil-thin branches to deposit their eggs. The wee nymphs are now falling to the ground, where they will burrow and feed, seeing daylight again as adults in 2038. A single female lays 20 or more eggs in each of multiple nests, and because there were countless billions of these insects in much of the Eastern United States, the damage is now widespread and evident.
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Species of oak seem a favored target, but the cicada is not picky. Small trees, big trees, young trees, old trees, native trees, exotics: It does not matter to a mother cicada driven to perpetuate her species.
The twig scarring consists of puncture wounds that often join up to form a long scar, usually on the underside of a branch. This damage has the capacity to disrupt the flow of sap to points beyond the damage. This is manifested in a number of ways. A branch might die back to 12 inches or so from its tip. The leaves in that section turn brown, and the wood becomes brittle and collapses, a process called flagging.
This is the most straightforward damage. The flagging branches will eventually fall off, and lateral growth from below will take over. If you can safely get to these dead tips, it is best to prune them off to a point where a healthy side shoot is growing. This will not only smarten up the tree but, more importantly, also leave a small, clean wound that will heal more quickly.
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The other cicada damage may take a while to reveal itself, either in stunted growth or drooping branches, a function of the wood growing normally on the upper side of the branch but not on the damaged side. If the top of the leader of a tree is dead or dying, you should cut that back to a healthy side shoot and allow that, in time, to turn skyward and become the new leader.
But no tree is worth risking your neck for. If you can’t safely attend to this pruning, the tree will grow through the damage. The speed of this self-repair can vary by tree type. According to a study in Maryland of the 1970 Brood X appearance, delayed branch dieback occurred over the following two years in such trees as black gum, purple-leaf plum, photinia, lilac and dogwood. But there was rapid, smooth healing in such trees as hornbeam and Southern magnolia.
As for the delayed effects, I would wait until next spring to see where new growth erupts, and cut out dead or damaged wood then.
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More immediately, cast an eye on your needled evergreens for a more pernicious and furtive insect named the bagworm. This is a moth in its larval form, and it spends the first half of the summer munching away, often in great numbers and to a point where it can ruin even mature plantings. This is because it hides in plain sight; that is, it builds a cocoon of dead needles, so it is effectively made invisible.
It starts off small in late spring, but by August, it is a large and rapacious creature beneath its magic cloak. You’ll probably find it on two popular screening plants, Leyland cypress and the arborvitae, especially the ubiquitous variety Green Giant. Defoliated conifers don’t rebound like deciduous trees and shrubs, so unchecked bagworms are bad news. “The reason the damage occurs is that people don’t know how to recognize them,” said Heather Zindash, an integrated pest management consultant based in Gaithersburg, Md.
Once you have trained your eye to recognize the brown shrouds, you may be appalled at how many you can see. The evergreens can be sprayed organically with Bt or spinosad, she said, until the pests reach a size when they stop feeding. They are still at a stage (just) when they can be sprayed. “In the next few weeks, they’ll get too large to control that way, but you can still physically remove them,” said Zindash, whose firm is the Soulful Gardener. Put on a glove, handpick them, bag them and throw them out.
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Another more persistent and more damaging creature than the cicada is the scale insect, actually a range of scale species that have their own preferences for ornamental woody plants and citrus. They are small, some the size of a grain of rice, but they can suck the life out of a plant in sufficient number. There are two basic types: The first covers itself in a conspicuous white, waxy coat, and the second lives beneath a dark and camouflaged carapace. Both are thus hard to control with contact insecticides.
Organic horticultural oils are available for use against scale; consumer products will probably be premixed to the correct lightweight solution. Follow the label instructions for timing, application and precautions.
The best defense against scale insects, in my experience, is a healthy plant. Give the tree or shrub the light, soil and moisture conditions it prefers, and your plant should be protected. Not all garden ornamentals have a 17-year break between attacks.
Tip of the Week
A dethatching rake is a great tool for removing thatch buildup in turf and for preparing a lawn for overseeding in a few weeks. Tackling small areas now will lessen the task come seeding season in September and October.
— Adrian Higgins
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