Football season is right around the corner, which means usually one other thing related to horticulture (No, not turf grass research at football fields). It's Fall Webworm time! Actually, in Texas, fall webworms can occur anytime from spring through the first frost and they are actually caterpillars and not worms. But the most common sightings seem to be in August through September. The "web" reference is extremely accurate. If you haven't seen them lately, you haven't been looking. The tell-tale sign of these pesky critters are numerous clumps of webbing in the leaves of the tree.
The fall webworm is the larval stage of a rather ordinary moth (Hyphantria cunea) which is mostly white with dark spots on the wings. The pale yellow or green caterpillar is about 1 inch long when grown with a broad dusty stripe running down the back bordered on each side by a yellow stripe. It is also covered with tufts of long whitish hairs. There are can be two to four generations per year depending on climate. The final generation is typically in the fall and is the worst infestation. That is where its name "fall webworm" was derived.
Fall webworms over-winter as pupae in silken cocoons under ground trash or on rough tree bark. Moths emerge from the cocoons and lay eggs in hair-covered masses on the underside of leaves. The newly hatched larvae (the actual webworm) begin feeding and spin webs over foliage in which they feed. Entire branches can be covered in webs and in severe infestations; the entire tree can be covered with several webs. The caterpillars eat the tender portions of the leaf, leaving the large veins and mid-rib. In the event of severe defoliation, the tree may be weakened possibly die.
These insects attack many tree species, but are most prevalent in mulberry, ash and pecan. Others trees including hackberry, peach, willow and red oak are vulnerable but severe infestations are not as common. Fall webworms may be controlled without using insecticides by thoroughly inspecting trees that have a history of severe infestations. In small trees, the egg masses and in larger trees, webs can be pruned out and destroyed. To be effective, the gardener must be dedicated to continually inspecting trees for new webs and/or egg masses to be removed.
If webs become too numerous or are out of reach for pruning, then insecticides may be needed. The environmentally safest ones are those containing Bacillus thuringensis, a bacterial derivative that is effective specifically against caterpillars. The toxins produced by the bacteria are ingested by the larvae and destroys the digestive tract of the caterpillar. The product, commonly called B.t. is marketed under the trade names of Dipel, Thuricide, Bactur, or Bioworm. It is safe to use on fruit and nut trees and on other caterpillar-like insects that cause damage, such as cabbage loopers that attack vegetables.
Other chemicals available for fruit and nut crops include carbaryl (Sevin) and malathion. Ornamental or trees can be sprayed with acephate (noted as Systemic Insecticides on some bottles). Complete coverage is needed for good control. A high pressure sprayer may be necessary for to reach the tops of trees or to penetrate dense webs. Ortho makes a tree & shrub sprayer, other nurseries carry a commonly known sprayer for trees called a Gilmore Sprayer, and still others have a "trombone" sprayer that is also designed to get higher into trees. It may be necessary to use a commercial tree spraying service for large trees or heavy infestations.
At the very least, these caterpillars will not normally "kill" a tree, rather make it "unsightly" at best (or worst, depending on your perspective). Insecticide labels change frequently. Make sure you read and follow the label directions on the container for use and precautions. Look at the crops labeled for use remembering that it is illegal to use an insecticide on crops not listed on the label. Also remember that the pesticide USER is ALWAYS responsible for the effects of pesticides on his own plants or household goods as well as problems caused by drift from his property to other properties or plants.
Remember though the headline of this story - Unsightly, Yes! Deadly, No!! You really don't have to do anything, if you so choose. But also remember that if you have a heavy infestation and you want to spray an insecticide of sorts, you will absolutely need to rip open the webs, so that the spray can make contact with the caterpillar. They use the webbing as a protective armor against birds and insecticides. Most people I now simply fashion home-made tools to do this ripping of the web before they start to spray. Long PVC piping with nails fashioned on the tip are a great home-made remedy. If they tree is not too tall,a simple leaf rake can do the trick.
Early recognition and control is the key to minimizing damage from fall webworms. So if webs start appearing in your trees, don't put off control measures. Attack those webworms quickly and reduce your frustrations before they get out of hand. This early line of defense will help you win the war against fall webworms.
