There may be limited options for root-absorbed insecticides that are legal for the general public to apply. You may need to investigate product labels to see which are usable in drench form, which include bagworms (or at least "caterpillars") in the list of pests controlled, and which do not use a neonicotinoid-class ingredient, as those are prohibited in Maryland except for use by certified pesticide applicators. The ideal treatment would use Bacillus thuringiensis (Bt) or perhaps entomopathogenic (insect-killing) nematodes, but these would need to be sprayed on, and thus applied by someone hired to make the application so they have the right equipment to reach tall trees. Spinosad is another active ingredient that may be useful with limited risk to other insects, but it too requires a spray application. Active ingredients that have been labeled for bagworm suppressing include abamectin, acetamiprid (a neonicotinoid), and emamectin benzoate, but not all might be available in drench form, or readily accessible to the general public.

Miri

To add onto Miri's answer, there are not many root drench products available for effectively treating bagworms and the only root drench products for bagworms I am aware of can only be applied by a licensed pesticide applicator. Also, drench applications are best applied in the spring when the tree is putting out new growth and moving nutrients upward from the roots (and thus moving the product upward into the new foliage).

As Miri said one of your best options is to apply Bacillus thuringiensis, subsp. Kurstaki (Bt) as a foliar spray as soon as possible while the bagworm caterpillars are still relatively small/young (vulnerable). Bt is a naturally occurring bacterium that can be sprayed onto landscape plants. The spores are ingested by the insect as they feed on plant foliage. Once the spores are ingested they release a toxin inside the insect’s gut, killing the insect within a few to several days. Bt is commercially available under the following common brand names: Dipel, Thuricide, and others. Thorough coverage of the plant foliage with Bt is key to effectively treating for bagworms. 

Bt would need to be re-sprayed/re-applied every 7-14 days. Since your plants are tall we would recommend contacting a professional. By late July the bagworm caterpillar will be mature and cannot be effectively treated with Bt or other pesticides. At that point physically removing bags and waiting until spring for chemical treatments would be best. Luckily bagworms will not cause any feeding damage in the late summer through winter, as the adults mate, lay eggs, and die off, while the eggs spend the winter months in the protective bags waiting to hatch come spring (around May). 

You're welcome.