"What is this webby mass on top of my River Birch, and will to come back?"
Late summer and fall is the season of fall webworm. Fall webworm attacks many hosts, over 85 known species of deciduous trees, including elm, hickory, pecan, plum, chokecherry, poplar, walnut and willow. In fact, almost all fruit, shade and ornamental trees, except conifers, can be affected by fall webworm. A similar insect, called Mimosa webworm, is very common on honeylocust.
Homeowners often spot fall webworm as they enlarge their silken webs in late summer. Adults of this native insect are white moths, with reddish-orange front legs and a 1.25 inch wingspan. Immature insects are pale yellowish caterpillars with red heads and reddish-brown spots. An alternate color variation among the larva is yellow-green caterpillars with black heads a broad dark stripe on the back and black spots. The caterpillars have many long, fine hairs on their backs. There are one to two generations per year in Nebraska.
Adult moths emerge in late spring or early summer and lay eggs in masses on the undersides of leaves. The larvae emerge 10-14 days later and begin feeding in groups within a small webbed mass of leaves at the ends of branches. The webbing provides protection from some predators and the caterpillars feed inside the web until all leaves are devoured, then additional leaves are encased in the web. Webbed areas of leaves grow larger as the caterpillars mature, becoming a messy, ugly eyesore as it is filled with shed skins, excrement and leaf fragments.
The first generation of caterpillars matures in about six weeks. Then they drop to the ground and enter the soil, where they pupate into adults and re-emerge to lay eggs for the second generation. Some larvae may pupae under loose bark, in leaf litter beneath the tree, or within the webbing. Caterpillars of the second generation hatch and feed from approximately early August through late September. Then once again, the mature caterpillars drop to the ground and enter the soil to overwinter. Because the insects overwinter beneath host plants, trees that have been attacked in the past will very likely have insects the following year, too.
Although unsightly, feeding by fall webworms is rarely seriously damaging to large trees; however, several years of defoliation for small ornamental trees can weaken them. The web impedes most insecticides from reaching the insects, unless you can catch it early. One of the best ways to get rid of them is by taking a rake and breaking up the web. Or you can try a heavy stream of water to break up the webbing.
Many of the caterpillars will be knocked out of the web onto the ground, and will be killed by predatory insects. Biological insecticides such as Bacillus thurengiensis, B.T., or Dipel are also effective. Other insecticides, such as permethrin and bifenthrin, will also provide good control. Thoroughly cover leaves next to the nest, and as the larvae ingest the insecticide they will be killed.