Using Natural Insect Repellents
There are several natural insect repellents that often kill cankerworms on contact, but also serves as a repellent by creating a bad taste for the critter and make it go someplace else to munch. Neem Oil Soap, Hot Pepper Wax, and Garlic Barrier all have this potential repellent capability when applied properly on the tops and the bottoms of leaves of the target plant. The repelling effect will last for several weeks to a month depending on the weather conditions. See the Natural Repellents in Yardener's Tool Shed.

Using Natural Insecticides
Neem oil soap is a natural insecticide that will kill cankerworm on contact and then repel new arrivals for about 2 weeks. Pyrethrum is made from chrysanthemum blossoms and is quite effective especially when applied directly to the pest. Light horticultural oil is effective in smothering [insect]. Go to the Natural Insecticides section in Yardener’s Tool Shed for details on each product.

Try Bacillus thuringiensus
As soon as you find cankerworms on your trees, spray them with Bacillus thuriengensis (BT) every 10 to 14 days until the pests are gone. If your trees are infested with eggs, begin spraying them once the flowers have finished blooming. Apply the Bt every two weeks for a month.