Question
Our 2 trees are about 38 years old and are very tall! We have had the web worm treated for the past 8 years and have had no problems, but it is expensive.
Is there a height at which the web worms no longer attack the tree? I am thinking that maybe we can stop the treatments next year, but am afraid the worms will return. Thank you.

Answer
This is called Mimosa web worm. No I am sorry there is no height that the web worms stop infesting. The population will vary each year and some years there will be very few. They have two generations per year (in June and Aug) So treatment should be made in early June and Aug to get the best results. Also rake up the leaves and destroy them this will cut the population down.

Spray with formulations of acephate, azadirachtin, Bacillus thuringiensis, bifenthrin (Bifenthrin Pro Multi-Insecticide, Onyx Insecticide, Talstar F, Talstar Lawn & Tree Flowable, Talstar GC Flowable, Talstar Nursery Flowable, and TalstarOne Multi-Insecticide only), carbaryl, chlorpyrifos (Dursban 50W only), cryolite, cyfluthrin, cyfluthrin and imidacloprid, deltamethrin, diflubenzuron, disulfoton, lambda-cyhalothrin, permethrin (Astro Insecticide and Permethrin Pro only), pyrethrins and piperonyl butoxide, spinosad, tau-fluvalinate, and trichlorfon are labeled for webworm management.
Management Hints: First generation adults appear in June and lay eggs on flowers and foliage. Manage first-generation larvae as they hatch from eggs in mid- to late June. Second-generation larvae may require treatment in August.

Here is a web link to more information on this insect  http://woodypests.cas.psu.edu/factsheets/insectfactsheets/html/Mimosa_Webworm.ht