Question
Hi,  we have 8 year old cottonwood that has been healthy up until this year.  We noticed a large web like thing on one of the limbs.  By the time we got it down, the limb had died.  We have not seen anymore web looking things but we keep loosing limbs slowly, one at a time.  We have noticed the bark has some black areas on it and is peeling/splitting a lot.  Any help is appreciated.  We love this tree so much!  We live on the Plains and trees are really hard to grow.  Thanks.

Answer
The web insect will not kill the tree but it sounds like you may have sun scald on the trunk.

Sunscald is another form of winter injury that can cause cracks and splits. Sunscald occurs when cells in the living tissue beneath the bark thaw out on sunny days. This occurs mainly on the south or west side of trunks and branches. These cells rupture when they re-freeze at night. The tree is injured when enough cells in a given area rupture. You'll notice the injury the following spring as a discolored, sunken area. Fungus infections often invade trees via sunscald injuries. Young, thin-barked trees are most susceptible to sunscald injury. These include maple, honey locust, linden, and mountain ash. Heavy pruning on neglected trees exposes sections of bark that have been protected from the sun's direct rays for years, predisposing them to sunscald injury.

You can reduce or eliminate sunscald injury on young trees by wrapping the trunks each fall with tree wrap paper. Do this every year until the bark begins to roughen. This may take only a few years on some trees, but more years on others. Prune trees that haven't been pruned for years in stages, not all at once. This will help prevent sunscald.

I would spray the area with an insecticide called Merit to prevent borers from entering the trunk. And fertilize the tree with 10-10-10 fertilizer at the rate of 1 lb per inch of trunk diameter scattered around the tree and watered in good. This will increase the health of the tree and allow the tree to better stave off any insect or fungi.
In general I would not be too concerned about the crack except for borer prevention