Some people are against any type of insecticide application regardless of the product used. Bt is toxic to all caterpillars and in some instances killing them would be of concern, especially if any rare moth or butterfly species are involved. If your neighbor objects to spraying, take precautions to avoid the problem and liability of spraying their land by mistake. FTC is a native defoliator of hardwoods trees, especially aspen and basswood. The first recorded outbreak in Minnesota occurred in the late 1870's and there have been five major outbreaks since 1933.
Outbreaks usually last three to six years and cause little or no permanent damage to the trees.
The survival of the forest is not threatened and therefore the forest does not need the protection afforded by pesticides. Trees generally refoliate by early July. It is also extremely difficult to obtain a level of control which will actually reduce populations in future years. Control programs can prolong local outbreaks. Controlling FTC on federal, state and county lands can't be justified biologically or economically.