|How can I prevent bag worms in Pecan trees that are over 20 feet tall?|
What you most likely are seeing is webworms. Bagworms form small elongated ?bags? about the size of your little finger or smaller. These bags, camouflaged with debris from the plants, are usually are found on evergreens, such as junipers. Webworms on the other hand form large loose web structures in the branches of trees. They absolutely love mulberry and are also fond of pecan as well as a number of other tree species.
The simplest way to deal with webworms is to take a long pole and destroy their protective webs. Repeat this every time you see that they have built more webbing. Without the protection, they are sitting duck for paper wasps and other predators.
An effective, organic spray option is to apply a spray containing B.t. to the leaves after disturbing the webs. B.t. is sold under many brand names including Dipel and Thuricide, and is most effective on young caterpillars. When they eat the leaves sprayed with B.t. the caterpillars will soon get sick, stop eating and die.
Spraying a home pecan tree can be a challenge since most home garden equipment won't provide adequate coverage of a tree. There are some hose end sprayers designed for trees that have a long nozzle that angles upward at about a 45 degree angle. These can reach fairly high (about 25 feet) with decent hose end water pressure. One such device is the Ortho Tree & Shrub Sprayette Hose End Sprayer. The next step up the line is a small motorized sprayer but these are much more expensive. That's why most people hire the work done.
The caterpillar control pesticide that is the least toxic and is also natural is B.t. (Bacillus thuringiensis kurstaki) and is sold in numerous product names including Dipel, Thuricide, Bio Worm Killer, and Greenlight BT Worm Killer. Another low toxicity, natural option is spinosad, which is in such products as Greenlight Spinosad, Monterey Garden Insect Spray, Bulls-Eye Bioinsecticide and Ferti-lome Borer, Bagworm, Leafminer and Tent Caterpillar Spray.
The webs prevent spray from reaching inside to the foliage where the caterpillars are feeding. You need to break up the webs with a long pole before spraying because the insects must feed on treated foliage in order to be controlled with these sprays.
Keep in mind when making broad applications of these products that they will also kill butterfly larvae (caterpillars) if they drift onto plants which are larval food sources. These products break down fairly quickly and must be reapplied as needed to maintain caterpillar control. If you choose to hire a professional to treat for webworms ask them if they can apply these or another similar product.