Friday, August 8, 2008

It's not fall yet, so why do I have fall webworms?

While I haven't been getting many questions on them, I thought I would address fall webworms since I've been seen tons of webbing in trees all over town. Webworms can attack over 88 species of plants, but are often seen on pecan trees in Texas. We have 2-4 generations each year that start in the spring and continue into the fall. Fall populations are often the most damaging, giving the name fall webworms.

Caterpillars, the immature state, are very pretty in my opinion. They are about an inch long when fully mature and are pale greenish-yellow with long tufts of hair projecting from their body. Adult moths are somewhat drab with whitish coloring and small, dark spots on the front wings.

When caterpillars emerge from the egg, they immediately begin to spin the webbing that is expanded to cover the tree's foliage. The caterpillars use the webbing to protect them from predators. They enclose the foliage that they feed on and will expand the web size when they run out of foliage to eat.

So, options for managing webworms really depends on how annoyed you may be with the webbing, how large the tree is and what kind of effort you want to give to manage the caterpillars. Egg masses can be pruned or picked off the tree (egg masses are on the underside of leaves and typically covered with hairs). Pruning webs out of the tree or opening the webbing with a stick or stream of water can also help to manage populations. Pruned webbing should be disposed of in a sealed, plastic bag or dunked in a bucket of soapy water to kill the insects. Opening the webbing can allow predators to enter the web and help manage the pests. You do not want to burn the webbing out of the tree as this can often cause damage to the tree as well as being dangerous.

Of course, there are also chemical options. Less-toxic options include using active ingredients such as Bacillus thuringiensis variety kurstaki, also known as Bt. This variety of Bt specifically targets caterpillars, but will not differentiate between "good" and "bad" caterpillars, so avoid drift. Spinosad which comes from a soil microorganism is another option. Both of these chemicals must be consumed for them to work properly, so good coverage of the foliage is key (hence, you must open the webbing to get the chemical to where the caterpillars are feeding!). These options work best on smaller caterpillars (less than 1/3 inch).

For management of larger caterpillars, contact kill chemicals work best. Some active ingredients to look for include permethrin, cyfluthrin, carbaryl or acephate. Again, the chemicals have to get into the webbing where the caterpillars are located or they will not work very well.
Of course, you can always choose to let the caterpillars do their thing. The trees and caterpillars have been here living together for many, many years and nature always seems to find a balance of things on it's own.
Also, don't expect webbing to disappear from the tree once the caterpillars are gone. You will either have to wait for a heavy rain or use a high pressure water stream to knock the web out.

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