Battling fall webworms

Flower & Garden Magazine, July-August, 1998 by David Wimberly

Something's pupating under your tree and in your garden. And soon the whole neighborhood will know. Sound disgusting? It,, is. It's the so-called "fall webworm," Hyphantria cunea.

These inch-long green or yellow caterpillars, bristling with silky hairs, are actually moth larvae. In summer and early autumn they weave expansive webs that bind together the ends of branches. They eat every leaf within reach, continually building larger webs until late autumn, when they pupate. Resembling dirty rags and filled with black droppings and wriggling worms, the webs ruin the aesthetics of any garden.

Hyphantria cunea attack many kinds of deciduous trees, shrubs and vines. Pecan trees are a favorite target in my home state of Alabama. If you live in the lower 48 states or southern Canada, it's a sure bet that webworms could find something to suit their appetite in your garden.

Besides aesthetic concerns, web-worms can seriously damage the harvest of fruit and nut trees. Not only can they quickly consume vast quantities of leaves that are needed for fruit production, but they also bind up the ripening fruits and nuts in their webs.

What's worse, webworms can seem impossible to fight. Their webs are often out of the reach of conventional sprayers; commercial spray equipment can result in a rain of pesticide over large areas of your garden. It's enough to ruin your appetite for, say, pecan pie.

To plan an effective schedule of prevention and control, it helps to understand the life cycle of Hyphantria cunea. In early spring, moths come fluttering up from cocoons carefully hidden in the bark of tree trunks and in ground debris. The adult moths are about 2 inches from wingtip to wingtip and are white spotted with brown. They lay eggs in clusters on the leaves of suitable host plants.

The eggs hatch within a week. Out come tiny caterpillars, the first generation of the growing season, spinning webs and eating leaves. This first infestation is so mild that sometimes the webs go unnoticed.

The first-generation caterpillars have eaten their fill by early summer. They form cocoons and pupate, usually in the bark, of a tree or underneath leaves or other debris on the ground. By midsummer they re-emerge as moths. A second generation follows -- this time larger and more destructive.

Because the webworms -- in one form or another -- are present year-around, it's possible to devise a year-round strategy for dealing with this pest. Here are some tactics that have worked for me:

* In winter or early spring, remove fallen leaves, ground debris and mulch, which may harbor overwintering webworm pupae. Replace the debris with fresh, pest-free mulch.

* Inspect susceptible plants for the greenish egg masses, which are typically laid on the undersides of leaves and are protected by a woolly or scaly covering. Eggs are deposited from late spring through fall. Remove any affected leaves and destroy them. This strategy requires time and sharp eyes and is obviously impractical for tall trees.

* Bacillus thuringiensis (Bt) is a microbial pesticide that can be applied as a dust or spray. It kills many types of caterpillars but has no effect on warm-blooded animals or bees.

With fall webworms, Bt is effective only if its application is properly timed. Once the caterpillars have enshrouded themselves in webs, they are more difficult to kill. Check on susceptible plants frequently, beginning in late spring, and apply Bt at the first sign of hatching webworms. Bt loses effectiveness after about two days, so it must be reapplied as long as more larvae are hatching.

Always use Bt with care, because it also can kill the larvae of non-pest moths and butterflies.

* When you see webs, clip the infested branches and burn them, or drown the larvae in a bucket of soapy water. A pole pruner with a lopper blade will help you reach webs within 16 feet or so of the ground.

* When I was a boy, we would torch the webworms. We would cut gigantic poles from a stand of bamboo and with wire would bind rags to the end of the poles. We'd then light them and raise our flaming torches to the webworm nests. If you choose to use a torch, take proper precautions by wearing protective goggles and a wide-brimmed canvas hat damp with water. Keep in mind you may also need a burning permit from your local fire department.

* Another type of tool works as well as the torch. Into the end of a long bamboo pole or a length of PVC pipe, drive two long nails at 90 degree angles to each other. Leave the ends of the nails protruding from the pole. Push the end of this tool into the center of a web and twist. The web will,catch on the nails and roll itself around the pole like cotton candy. Most of the caterpillars will be rolled up with the web. Again, be sure to wear eye protection and a hat to guard against falling debris.

The trick to preventing unchecked rampages of fall webworms is attacking the pest when it is most vulnerable. A little work in winter, spring and summer will go a long way toward reducing the damaging infestations of autumn.