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Garden Q & A: Caterpillars weave web of destruction in trees

Saturday, August 26, 2000

By Sandy Feather, Penn State Cooperative Extension Agent

Q. I have two weeping cherry trees in my yard that have cobwebs on their branches. On closer examination, I noticed caterpillars in the cobwebs that are defoliating the affected branches. What are they, and how do I get rid of them?

A. They are the nests of fall webworms. These caterpillars usually begin their activity in late June or early July. At first, their webs are so small they escape notice. Fall webworms build their nests on the ends of branches and feed within the web, enlarging it as necessary to obtain more food. They feed on a wide variety of deciduous trees.

Small infestations within reach can easily be pruned off the tree and disposed of in the trash. Larger infestations, or those out of reach, can be controlled with Bacillus thuringiensis (Bt) when the caterpillars are still small, say under a half-inch long. Bt is a bacterial disease that infects caterpillars and is very safe for use around children and pets. You must thoroughly spray the foliage inside the web and just beyond it, because the caterpillars must ingest Bt for it to be effective. Once the caterpillars get larger, Bt does not provide adequate control. Orthene (acephate), BioNeem (azadirachtin), Sevin (carbaryl) or diazinon are more effective on larger caterpillars. All of these products are labeled to control fall webworms on a wide variety of plants. Be certain that the product you choose is labeled for application to the affected plant.

It is very difficult to spray through the large, tough webs you see in late summer. The webs will remain until they wither away, even if you get the caterpillars under control. Next year, start scouting the affected plants in your yard in late June or early July so you can take steps to control them while the caterpillars and webs are still small.

You do have the option of living with them, particularly if they are affecting only a branch or two on a large tree. Fall webworms do their worst damage late enough in the season that the tree's buds for next year are already set. An otherwise healthy tree will not sustain life-threatening damage. If an affected tree is already stressed by drought, disease or damage from other insects, it is a good idea to control them. Also, newly planted, small trees should be protected from complete defoliation.

People often confuse fall webworms with Eastern tent caterpillars or gypsy moths. Eastern tent caterpillars build their webs in the crotches of trees -- especially cherry, crabapple and apple trees -- in the spring, just as trees are starting to leaf out. Unlike fall webworm, Eastern tent caterpillars leave their nests to feed and return in the evening or when threatened. The same products labeled for control of fall webworm are effective against Eastern tent caterpillars. Be sure to spray or mechanically remove their webs in the evening when they have returned to their nests for the night.

Gypsy moth larvae are also active in the spring, just as oak trees are leafing out. They do not spin webs at all. They spin threads of silk that carry the small larvae from tree to tree on the wind. That is why they are called gypsy moths. Bt, Orthene (acephate) and Sevin (carbaryl) are labeled to control gypsy moth larvae. Be sure to hire a certified arborist to spray very large trees. They have the training and equipment to do the job safely and thoroughly.

Q. I have 12 sweet pepper plants that have only produced one pepper all season. They are very healthy and stand 2 to 3 feet tall. They are planted in partial shade. If I put them out in full sun, they look as if they are dying. How can I solve this problem?

A. Peppers will not produce well in the shade, even partial shade. Most fruit-producing vegetables -- peppers, tomatoes, cucumbers, squash, eggplant, melons, okra, etc. -- require full sun to be productive.

Your plants may look rough when you plant them out in full sun because they have not been hardened off properly. Hardening off is the process of gradually getting young transplants prepared for life outdoors after being pampered in a greenhouse or under lights in your house. If transplants are not hardened off, they may get sunburned or windburned when planted out into the garden. They can die from the damage. Do not assume that transplants have been hardened off when you purchase them.

Hardening off should be started two weeks before you want to plant them into the garden. The process is intended to slow growth and allow succulent tissues to harden off to the point where they are not burned by sun and wind. Reduce the frequency of watering, but never to the point of wilting. You should also reduce or eliminate fertilization until your plants are planted out in the garden. Chose a shaded spot that is protected from the wind and start putting them out for several hours in the morning. Lengthen the amount of time they spend outside every day, and gradually begin to expose them to the sun. Start with morning sun and allow them to remain in the sun for longer periods of time until they can take full sun all day. Then you can plant them out in the sun and they should get off to a good start. They will be far more productive than the ones growing in partial shade.

You did not mention your fertilization practices, but it is important not to place granular fertilizer in the bottom of the planting hole. The fertilizer can burn the tender, newly emerging roots to death. This would result in the plants wilting, especially in the sun. You do say your plants are growing vigorously. Peppers are light feeders, which means that they do not require a lot of nitrogen fertilizer. It is possible to over-apply nitrogen and have peppers put on vegetative growth at the expense of producing peppers. It is wise to make fertilizer applications on the basis of soil test results. Penn State soil test kits are available from your county extension office for a nominal fee.

Send questions to Sandy Feather by e-mail at or by regular mail c/o Penn State Cooperative Extension, 400 N. Lexington St., Pittsburgh 15208. Due to volume of mail, personal replies cannot be provided.

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