AUGUST
1998

 

 

 

By
Tom Turpin
 
Professor of
Entomology
Purdue University

 

 

 

 

 

8-27-98

Webs in Trees Aren't Work of Halloween Spiders

During August and September, some hardwood trees in many parts of Canada and the United States are adorned with silk webs. These webs can be so numerous that it appears Mother Nature is beginning to decorate for the Halloween season.

But the webs aren't very decorative, and they aren't the product of some hard-working spider trying to capture an insect meal. These webs are the work of caterpillars-caterpillars that appropriately are called fall webworms.

Fall webworms are the immatures of a moth. These caterpillars feed on the leaves of many types of trees. More than 100 species of hardwoods, including most fruit and shade trees, can become food to webworms.

There are two generations of webworms per year. The first generation occurs in the spring as trees leaf out. The overwintered female moth will lay a group of eggs on the tree leaves. The eggs hatch and the little caterpillars begin to eat leaves and spin a protective cover of silk.

Webworm caterpillars are very hairy and are pale yellow in color with black spots. All of the caterpillars from the batch of eggs band together in the same web. Sometimes, the web will cover several branches of the tree. The web also will contain numerous black pellets-the manure of the caterpillars.     

The caterpillar stage lasts from four to six weeks. At this time, the caterpillars stop eating and crawl down the tree where they hide in the soil or leaf litter to go through the pupal stage. After this, they emerge as moths and lay eggs. The eggs hatch and a second generation of caterpillars begins to feed during the fall months.

Some scientists believe that the silk web helps protect the caterpillars from wind, rain and predators, such as other insects and birds. While the web may benefit the insect, it is unsightly and many tree owners want to eliminate it.

In general, the feeding of the webworms is unlikely to do permanent harm to mature trees. Newly transplanted trees, however, might be more susceptible to damage from the caterpillar feeding. Control can be achieved with an insecticide labeled for that purpose.

However, in some cases, the worms can be removed from the tree by pruning the portion of the branch with the web. The web and the worms can be destroyed. A fire will do the job nicely!

Some people like to destroy the offending webworms themselves. So, they do a version of the bug stomp, using shoe leather to smash the caterpillars in the web.

The control method selected, or whether control is used at all, has nothing to do with the presence of webworms next year. Webworms, like many other insects, seem to occur in cycles with several years of low numbers followed by an outbreak of a year or two when populations are high. It is these years when every tree seems to be adorned with the webs.

If you are worried about webworms in a tree, think of it this way: You already have your Halloween decorations in place!

 

Writer: Tom Turpin
Editor: Olivia Maddox