In my opinion, webworms seem to be appearing earlier this year.
Historically we’ve referred to them as “fall webworms,” but they can actually crop up anytime, from spring through the first frost. And they are really caterpillars, not worms.
Their “webs” are a tell-tale sign that the pesky critters are already turning up in the branches of fruit, nut, and shade trees. The picture above of a redbud tree comes from my buddy, Frank Chambers. I shot the one below of a neighbor’s mulberry tree.
The webworm is the larval stage of the rather ordinary Hyphantria cunea moth (inset in the photo above), which is mostly white with dark spots on the wings. The pale yellow or green caterpillar is about an 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 can be two to four generations of the critters per year, depending on climate. The final generation, typically in the fall, is usually the worst infestation. That’s where the "fall webworm" got its name.
Webworms overwinter as pupae in silken cocoons under tree trash or on rough tree bark. When moths emerge from the cocoons, they lay eggs in hair-covered masses on the underside of leaves. Once hatched, the larvae (the actual webworm) begin feeding, spinning webs over foliage as they eat. Entire branches can be covered in webs, and in severe infestations the tree can be fully covered. The caterpillars eat the tender portions of a leaf, leaving only the large veins and midrib.
These insects attack many tree species, but they are most prevalent in mulberry, ash and pecan, which tend to experience the most overwhelming infestations. Others, including hackberry, peach, willow and red oak, are also vulnerable, but severe infestations on them are not common in our area.
Fall webworms may be controlled without insecticides by thoroughly inspecting trees that have a history of severe infestations. In small trees, the egg masses can be pruned away and destroyed. In larger trees, webs can be pruned out. To be effective, though, you must be dedicated to continually inspecting trees for new webs or egg masses to remove.
Realistically, though, most people don’t notice them until there’s a web. To treat them once detected, you can scrape open the webbing of those you can reach with a hoe or rake, and spray with them with any liquid insecticide - bifenthrin, Sevin, malathion or Triazicide will work. The webbing protects the critters from sprays, so it’s important to break it open.
Next, it’s important to spray all the other green leafs with the organic and caterpillar-specific insect control known as Bacillus thuringiensis – Bt -- a bacterial derivative that is effective specifically against caterpillars and worms. Bt is found commercially as Dipel, Thuricide, Bactur, or Bioworm. It is safe to use on fruit and nut trees, and it also works on other caterpillars, such as cabbage loopers that attack vegetables.
Insecticide labels change frequently, so be sure to carefully read and follow directions on the container. Note the crops for which it is intended, and remember that it is illegal to use an insecticide on crops not specified on the label.
Although these webworms will not normally kill a tree, they will make it rather unsightly. Early recognition and control are the key to minimizing their damage. So if webs start appearing in your trees, don't put off control measures. Attack webworms quickly, before they get out of hand.