Question
One of my large crab apple trees, and my weeping cherry tree have whole branches that have been enveloped by a webby substance, and it has killed these branches. What is this and how should I treat it? I live in W.MA in the Berkshires, and I haven't ever seen this before. Are all my trees in danger?

Answer
Sounds like Fall Webworms. These caterpillars construct a web over the foliage in the late summer or early fall and defoliate the tree. They do not kill the tree but will reduce the pecan crop and make the tree look bad. These insects a cyclic in nature and the population will vary greatly each year --some years there will be a lot of them and other none.
The webworms drop into the mulch or soil beneath the plant to pupate over winter. The adult white moth emerges in May to July depending on the climate and lay masses of white eggs usually on the underside of leaves. In a few days hatching occurs and the larvae begin to spin silken webs over the foliage, covering the area that has been devoured. Usually the larvae of the first generation are active in June and July and the second generation in August and September. The nests produced by the second generation are usually more numerous than the first.

The fall webworm tents are ugly but they seldom kill mature trees and plants. Most plants will tolerate leaf loss after midsummer and will come back the following year. There are several methods for control of the webworms. (1) A number of native insects and animals prey on webworms. The webs can be opened with a stick, rake or stream of water to allow the natural predators inside. (2) Remove tents by pruning the end of the branch and destroy. (3) Though seldom required, several insecticides can be used.

Microbial Bt will attack small caterpillars or Diazinon, Sevin or Orthene for larger tents. The hardest part will be getting the spray into the foliage of large tree. Sprays will probably do more harm to the beneficial insects than to the webworms. Never set fire to the tents that are in the tree. This may damage the tree and start a fire under dry conditions.

The best control is to remove and destroy the tents as soon as you notice them. Inspection of your landscape throughout the summer for insects and diseases will provide an opportunity for early controls.

Another thing that can be done on tall tree is to spray the trunk with Orthene and when the caterpillars crawl down the trunk to pupate they are killed.

Here is a web link to information on Fall Webworms and pictures of the webs so you can make sure this is what you have. http://www.hgtv.com/hgtv/gl_diseases_pests_insects/article/0,1785,HGTV_3580_1372...