Question
A friend at work told me she has a very large whitish, cottony nest of some kind in one of her trees.  She described it being in the outer most limbs and leaves area of the tree and not near the trunk/fork.  She wanted to cut it down and burn it.  I told her it was probably a spider or moth nest and that it was most likely harmless, and she should leave it be.  This sparked a big controversy in our office.  Another co-worker said it was a gypsy moth nest which produce moths that get in the house and eat  clothes.  Another co-worker said it was a moth nest, but the kind that are good for the environment.  Another said it was spiders.  What exactly is it and should she go ahead and destroy it?

Answer
It is Fall Web worm.

The fall webworm, Hyphantria cunea (Drury), is most often discovered when the unsightly, light gray, silken webs on the trees in late summer and early fall are observed. Webworms enclose leaves and small branches in their nests,

This pest usually eats leaves late in the season and the nests are generally concentrated to limited areas. Because of this, little real damage is done to most trees. However, the nests can look very unsightly and multiple generations in long summers can lead to significant defoliation.

Because the nests are only found on the ends of tree branches they are easily noticed. Webworm feeding cause no health danger to the tree, it only lowers the overall beauty of the tree because their nests are ugly. Remember that tree leaves are most efficient at producing food during the first few weeks after emerging in spring. As the season progresses into fall the leaves produce less and lesser food but have already stored enough food for next years leaf production and other growth. Leaf loss is less important in late summer and fall.

Though the webs are very unsightly, damage to most trees is considered to be insignificant. However, in southern states where several generations of attack can severely defoliate trees, control measures are needed. This pest tends to go through periodic population explosions. Outbreaks every four to seven years may last for two to three years and then natural control agents greatly reduce the activity.

Mechanical Control - Removal of Nests - Small nests can be pruned out of small to medium trees. Monitor trees early to detect the nests when only several leaves are involved. These small nests can be easily crushed. Do not burn or torch the nests in trees as this may do additional damage to the tree.

Biological Control - Apply Bacillus thuringiensis (Bt) - The bacterial insecticide, Bt, is quite effective against fall webworms if it is applied when the larvae are small. Use formulations with UV protectants and thoroughly cover leaves next to nests. As these leaves are incorporated into the nest and eaten, the Bt will be ingested. Bt will work but the caterpillars will need to feed on the insecticide covered leaves to be killed.

Contact insecticides are sprayed on the adjacent foliage and when the caterpillars crawl on the leaves they are killed or if the spray gets on the insect it is killed. These work faster than Bt. Recommended insecticides are Orthene, bifenthrin (Talstar Lawn&Tree)and liquid carbaryl (Sevin). Check with your local nursery for these for control of fall webworm.

These products can be sprayed by the homeowner with pump up type hand sprayers if the trees are not too large. For tall trees either contract with a pesticide operator or do nothing realizing that the tree will not be killed but may look not too pleasing with the webs in the foliage.