We have a row of Lealand Pines. There is damage to 4 of the pines. In our neighborhood I have seen many with this type damage. I found 3 bag worms, now removed. I have looked carefully for other insects. Nothing visable. The damage is on the south east side of the pine. They are in full sun.
Carroll County Maryland
Your trees look like a type of arborvitae. This looks like classic deer damage. The
Deer love to browse on arborvitae when food is scarce. Nothing can be done to help the arborvitae develop new growth in the defoliated areas. However, if possible, try to prevent additional damage.
Once deer establish a pattern of behavior, it is very hard to break. There are no easy answers. Consider fencing or tree protectors. Repellents wear away in the elements and deer become used to them.
The extent of damage determines if new growth is likely, Areas that are completely defoliated (no green foliage present) will probably not fill back in. Areas that retain some green foliage are more likely to fill back in but you will have to provide protection.
Here is more on deer and management including deer resistant ornamentals https://extension.umd.edu/hgic/topics/deer