Gardener: If it's fall, it must be the fall webworm

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No doubt you've heard of at least one of the following: red imported fire ant, Japanese beetle or the emerald ash borer. They are exotic, invasive insects that are causing great harm in the United States yet rarely get noticed in their native land. Non-native, invasive pests are a popular topic of discussion in the fight to protect ecosystems and biodiversity. Yet other countries are dealing with similar issues and some of their biggest pests come from the United States.

Our native fall webworm has caused great concern in China, where it has no natural enemies. Even some bird species will not touch the worms. They were first found there in 1979, having hitchhiked on ships into Chinese ports. Sometimes we are so focused on the invasiveness of foreign species here that we fail to realize that our natives can be as dangerous on other continents. Here in the states, the webworm rarely causes more than cosmetic damage.

By now, you are seeing those masses of spidery webs engulfing the tips of tree branches.

The fall webworm (Hyphantria cunea), or Drury, is the larval form of the tiger moth. The tiger moth is about 2 inches across from wingtip to wingtip. Its color can range from pure white to heavily speckled with brown. It is native to North America and is distributed throughout the United States and Canada.

Overwintering in leaf debris and mulch, the adult moth appears in May through July. She lays about 200 to 500 eggs in a greenish mass underneath the leaves of deciduous trees in late spring through fall. A wooly or scaly coating protects the eggs. Depending on the climate, one to four generations may be produced. The eggs hatch in a few days, and the larvae begin spinning webs around the leaves as they eat. They do not leave the protection of the web while they are eating and growing. Once they have had their fill, by midsummer, they form cocoons in the bark of trees and in ground debris. This first generation is small and generally goes unnoticed.

The second generation that follows is more obvious, larger and more destructive.

In much of America, there are two generations. The caterpillars are yellowish to greenish, covered with long, silky gray hairs, with a dusky stripe down the back and a yellow stripe on either side. They may be either black or redheaded. Full-grown larvae can reach an inch in length.

They only pose a minor threat as a forest pest, but shade trees and ornamentals can be heavily defoliated. Fall webworms feed on about 100 different species of deciduous trees but rarely on conifers. In the eastern United States, trees that are targeted include: pecan, walnut, American elm, hickory, fruit trees and some maples. In some areas, persimmon and sweet gum are also affected. In the western areas of the country, willow, cottonwood and fruit trees are favorites. In most cases, the defoliation that occurs will not materially damage the trees, because they will lose their leaves anyway. Only in severe cases is the infestation so bad the tree cannot recover. But, in the case of pecan and fruit trees, leaves are more critical for fruit production, and the ripening fruit can become bound up in the webs, substantially reducing the yield.

Homeowners have some control over populations of fall webworms. A thorough late-fall or early-spring cleanup of leaf litter and mulch will help eliminate many of the cocoons. Then determine your tolerance for the eventual occurrence of webs. Opening up the webs that you can reach makes the larvae available to birds, so you can let nature take its course. A spray or dusting of BT (Bacillus thuringiensis), while not harmful to wildlife, bees and humans, will kill non-harmful caterpillars and butterflies, and should be used with caution. The web must be open in order to receive the spray. And there are other, more non-selective pesticides. However, if you feel the need to intervene, I encourage you to use one of the above methods, as they are the least toxic means of control.

(Joe Lamp'l, host of "GardenSMART" on PBS, is a Master Gardener and author. For more information, visit www.joegardener.com. For more stories, visit scrippsnews.com.)

(Distributed by Scripps Howard News Service, http://www.scrippsnews.com)

THE GARDENER WITHIN