Fall Webworms are showing up in cottonwoods throughout western Colorado. This insect overwinters as a pupa in debris at the base of trees with the adults emerging, mating, and laying eggs in late summer on the twigs and branches where the caterpillars form their tents.
While cutting off the branches and burning the tents might seem like a good idea there are only so many branches you can remove without causing root damage. Cutting off branches also creates wounds through which pathogens can invade. Burning the tents while still on the tree is likewise not recommended as the damaged tissue can be highly susceptible to disease infection and insect attack. It would be better to let these caterpillars continue to feed than to remove or burn branches to control them. The tents are unsightly but unless the tree is severely stressed from some other malady, fall webworm infested trees most likely are not going to suffer from these beasties feasting away on a few leaves.
A spray of Bacillus thuringiensis (BT) and Pyganic are organic options. Pyrethroids such as permethrin, cyfluthrin and esfenvalerate are also recommended. Adding a non-ionic surfactant or a couple drops of liquid dishwashing soap per gallon of spray helps the spray penetrate the tent and reach the caterpillars within. Aspens are particularly sensitive to soaps and liquid concentrate sprays so test a small patch of leaves a day or so prior to spraying the complete tree to determine if the spray is phytotoxic. Or, leave out the soap.
Imidacloprid does not work on this insect as it is not effective on Lepidopteras (moths and butterflies).
The BT strains that work on lepidopteras are aizawai and kurstaki. See the web page at http://www.coopext.colostate.edu/TRA/PLANTS/btstrain.shtml for more info on the various strains of BT and the insects they are effective on.
The PyGanic label can be found at http://www.mgk.com/~/media/com/Files/Product-Labels/Branded%20Products/PyGanic_5_0_Specimen_Label.ashx?dl=true&dmc=1
Dr. Cranshaw’s publication on tent-forming caterpillars is at http://www.ext.colostate.edu/pubs/insect/05583.html.
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