Boerne Chapter

Webworms are coming…Learn to Combat Them

by Mark Peterson, Guest Columnist – Boerne Chapter, Native Plant Society of Texas
Published in Boerne Star on April 27, 2015

Webworms in tree leaves
Webworms eating leaf

Soon, those ugly, nasty webworms will be insidiously  attacking our pecans, mulberries, and fruit trees.  One day, the trees are full and green; the next, sticky nests abound.  What is a homeowner to do to stop this multi-legged menace?
First, a little background information regarding the webworm is in order.  The webworm, a native to North America, is in the Lepidoptera or Butterfly family and its scientific name is Hyphantria cunea.  Researchers have discovered two distinct races: a red-headed one and black-headed one.  These races differ in their lifecycle, markings, food preferences and habits, but both make silk nests at the terminal branches of their host trees, although the black-headed race forms a rather flimsy web, whereas the red-headed forms one larger and more rigid.
A webworm caterpillar passes through as many as 11 developmental stages or “instars”.  During each instar, feeding occurs within a web made of silk produced by the caterpillars.  Depending on the climate, one to four generations of the webworm occur per year.  Two generations per year are generally the norm for the San Antonio area.

Two interesting characteristics of webworms are, first, that when alarmed all the caterpillars in a nest will jerk in unison, most likely as a defense mechanism, and, second, that its common name, i.e. fall webworm, is an apparent misnomer.  Coming from Michigan where the caterpillar appears only in September, I learned H. cunea as the fall webworm.  Upon arriving in San Antonio, however, I realized that fall webworm does not necessarily have to appear in the Fall; it may appear in May or June.  When scrutinizing a garden or insect reference book, just remember that the fall webworm mentioned corresponds to our June webworm.  Obviously, this is another case of Yankee snobbery.

When contemplating potential control measures, three basic alternatives come to mind. First, a homeowner may simply prune out the nest and destroy the larvae.  This is effective and easy for two reasons.  Webworms build their nest around their food supply; they do not go looking for food.  By destroying the nest, the homeowner eliminates most, if not all, the caterpillars in one fell swoop.  The other reason is that webworm nests are generally located at the branch terminals of the canopy, thereby allowing the homeowner easy access in removing the infested portion.

The second alternative is to break open the nest either by attaching a nozzle to your garden hose and blasting the nest to bits or by swatting it with a long, stout stick, pretending its a pinata.  The nest serves as protection against predators and parasites. Once that protection is removed, the webworm’s natural enemies, such as wasps and birds, move in to feed.  Unfortunately, this is the least effective alternative in terms of eliminating the entire population.  On the other hand, it is an option for those anxious about the third control alternative – insecticides.

Insecticides are composed of either biological or chemical ingredients.  The most common biological agent is a bacterium, Bacillus thuringiensis, which is non-toxic to everything but caterpillars.  Bacillus thuringiensis or “Bt”, its common sobriquet, liquifies a webworm’s digestive tract once the caterpillar has ingested treated leaves.  Bt is distributed under several product names including Thuricide, Dipel and Bio-worm.  Despite rumors to the contrary, chemical based insecticides are safe when used appropriately.  Always read the label carefully.  Whether using biological or chemical insecticides, a qualified arborist or pest control professional should be contacted when treating large trees.

Finally, webworms do not generally pose a threat to a tree’s survival.  Usually more of nuisance than life-threatening, they themselves cannot kill a tree outright.  Only repeated severe infestations would adversely affect a tree’s health. In short, don’t lose any sleep over these creatures – I don’t.

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