Gall-forming insects attacking Hickory usually do not cause serious damage to the tree, despite the ground often becoming littered with fallen infested foliage. There are at least two related gall-forming aphid-like pests common on Hickory, called Phylloxera. They have complex life cycles, but only have one main point of vulnerability, when one generation is outside of the protective gall itself. Once galls are visible, nothing can be done for the rest of that growing season that will have any impact.

The Astro may not have been applied properly for control of this pest, nor does it appear to be labeled for its control, so it doesn't seem to be the best choice. Timing of bark sprays needs to be in the tree's dormant season, and even so, horticultural oil is likely more effective in this case. The active ingredient in Astro is permethrin, which is not systemic - meaning, it does not get absorbed into plant tissues to affect insects exposed to it after treatment, nor does it impact pests that are shielded from direct contact, as those inside galls would be.

Insecticide treatments of trees bearing galls are almost never warranted, and trees tend to rebound just fine after shedding affected foliage. In some cases, fertilization of the affected tree might improve its vigor, though this should be done with caution as sometimes extra nutrients end up supporting the pest insect as much as it does the tree.

Here is more information about this insect:

https://content.ces.ncsu.edu/phylloxera-on-hickory-and-pecan

https://bygl.osu.edu/node/302

https://bugoftheweek.squarespace.com/blog/2013/1/27/gall-darn-it-gall-insects-on-hickory-oak-and-elm-iphylloxera-caryaecaulis-andricus-palustris-colopha-ulmicolai

We recommend raking-up fallen Hickory leaves and disposing of them; do not compost. Repeat this in autumn as well. If you are considering fertilization, have a soil test performed first so you know what nutrients are not needed: https://extension.umd.edu/resource/soil-testing-and-soil-testing-labs

Limbs should not have been killed as a result of Astro treatment or even the Phylloxera. They may be bare temporarily as their leaves have shed, or they have other unrelated issues causing sapwood death, including root stress, wood borers, herbicide exposure, or pathogen infection. Assessment by a certified arborist is recommended and should help illuminate any possible causes of tree stress or injury. Some can be remedied and others cannot, but it can help clarify what interventions to prioritize. You can use either of the links below to search for qualified arborists.

https://www.treesaregood.org/findanarborist/arboristsearch

https://dnrweb.dnr.state.md.us/forests/tree_expert_search.asp

Miri