Webworms can be scary on trees, shrubs

Fall webworm on dwarf bald cypress.

October is the month of mischief. As All Hallows’ Eve approaches, toilet paper seems to grow on trees, plastic spiders make their homes on front porches and fungi fill the empty heads of jack-o’-lanterns.

Insects even join the fun, wrapping silken nests around the ends of branches. Web-spinning caterpillars hide inside the nests and devour the leaves.

They won’t kill the tree but can make it look like Halloween arrived as early as July. I’m not a fan of their work. The problem, for most homeowners, isn’t the insect or the missing foliage but the web they leave behind.

The main culprit this time of the year is the fall webworm. The name implies they appear in fall, but in the Lowcountry they begin laying eggs in early summer. In most cases, the adult flits about unnoticed. It’s an attractive white moth that lay eggs on favorite trees.

While some insects can be particular about what they eat, fall webworm caterpillars can feed on more than 80 species of trees.

The result of their egg-laying often will result in only a nest or two in a river birch or wax myrtle. However, pecan, walnut and some fruit trees can be covered. Pecans, in particular, are one of their favorites. A large pecan tree can become an enormous web and completely defoliated in midsummer.

Fall webworms start a web at the end of the branch. It usually goes unnoticed until it seems to engulf the branch. The nest is tightly woven and provides protection from predators while the caterpillars feed inside it.

The webbing is littered with cast-off exoskeletons and specks of excrement. The nest is expanded to draw more leaves inside. It takes about six weeks from the egg hatch for the caterpillars to emerge as an adult. In fact, many nests already have been vacated but will continue to look unsightly for many weeks.

When possible, prune off the branch and destroy the nest. They burn quite easily. However, most nests are too high to reach. Pesticides would need to be applied before the nest gets started because it won’t penetrate the webbing.

Timing that application can be very difficult. Systemic insecticides, ones that are absorbed by the tree and translocated to the leaves, can be more effective and have a longer residual effect.

However, systemics can be expensive and require a trained technician to apply. Even then, the webbing will remain, and that’s the real problem. Left alone, the tree will tolerate the feeding and the webbing eventually will fade.

Another web-spinning caterpillar is the Eastern tent caterpillar that appears in spring and builds a nest at the base of the branch instead of the end of it. Toward the end of spring, you’ll see them crawling on the driveway or deck looking for a place to pupate.

In the landscape, they mostly go unnoticed and appear only in spring. The treatments are the same as fall webworm although it’s easier to scoop the nest out of the branching crotch and destroy. And, yeah, it’s pretty gross.

Recently, rosemary webworms have been found on rosemary shrubs. These are a bit more insidious, wrapping small webbing around the branches and consuming the foliage.

The caterpillars are much smaller and difficult to locate in the webbing. They also can kill the host, so it’s important to remove them when sighted. I’ve noticed them appearing at the end of August or early September. It also seems they have more of a preference for certain rosemary cultivars, such as prostrate rosemary.

Frequently, webbing can be spiders feeding on insects. Most often, it seems they are present on aphid-infested crape myrtles. While they’re not hurting the tree, the webbing can be unsightly.

One last harmless webmaker is the bark lice. Tree trunks appears to be wrapped in silk while these ant-size insects crawl beneath harmlessly feeding on organic material. Of all the webs, this one offends me the least and makes for a great Halloween prop.

Tony Bertauski is a horticulture instructor at Trident Technical College. To give feedback, email him at tony.bertauski@tridenttech.edu.