Answer: The grub like bugs are probably maggots of the husk fly, an occasional pest on these walnuts. One way to reduce the population is to rake up (or place netting beneath the tree and collect them on that) and remove the walnuts as soon as they fall ) thus preventing the pest from remaining in the vicinity to overwinter and reappearing next year.
Another option would be to have the tree sprayed in the spring at bloom time to prevent the nuts from forming at all. (You would need to consult with your county extension as to which product to use and as to the best timing for applying it.)
I am offering these suggestions because it is usually a good idea to think long and hard before removing a large and healthy tree. In many cases the replacement will take many years to reach a comparable size and provide similar benefits to the landscape. Also, the new plant will have to contend with soil depleted of nutrients over the years by the growth of the previous tree.
Finally, in the case of walnuts, there is a possibility that the Carpathian walnut was grafted onto a black walnut root stock. (It is also possible it was seed grown.) The roots of black walnut (Juglans nigra) produce a substance known as juglone. Many plants such as tomato, potato, blackberry, blueberry, azalea, mountain laurel, rhododendron, red pine and apple may be injured or killed within one to two months of growth within the root zone of these trees. The toxic zone from a mature tree occurs on average in a 50 to 60 foot radius from the trunk, but can be up to 80 feet. The juglone remains in the soil after the tree is removed and can be a problem for years afterward when replanting.
Finally, if you decide to remove the trees, you will need to consult with a professional tree service as to the process they recommend. A homeowner would not have the equipment and skill needed to take down a large tree safely.
In most cases the wood from a landscape tree is not suitable for other uses such as furniture lumber and ends up being hauled away or used as firewood. Stumps can be ground or left in place to eventually rot away over the years.
As to what to plant in their place, you will need to determine the growing conditions and make a selection based on what will have the best chance of thriving in that location. Your county extension and/or local nurseries should be able to make some suggestions.
In any case, you will not be able to plant them in precisely the same location due to the stump disruption and soil exhaustion.
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