Any idea how to repair this fence post? I was thinking of putting on one of those decorative caps but wondering if there is some sort of resin I should put in the hole? Not even sure how this happened. The post still feels solid. The exterminator says it is not insects that caused it. Maybe some frost/thaw cycles? I have a few other fence posts on the property with the same issue.

Any idea how to repair this fence post? I was thinking of putting on one of those decorative caps but wondering if there is some sort of resin I should put in the hole? Not even sure how this happened. The post still feels solid. The exterminator says it is not insects that caused it. Maybe some frost/thaw cycles? I have a few other fence posts on the property with the same issue.
If it’s just on top like that it looks like freeze/thaw or rot from water intrusion. A cap should slow/prevent further damage. To repair the existing damage (if it’s severe enough to worry you), spackle some structural wood filler like this stuff on it and you should be good to go:
https://www.amazon.com/Products-16333-WoodyTwo-Part-Repair-Epoxy/dp/B000H5O7Y8/ref=sr_1_1?hvadid=77721781959930&hvbmt=be&hvdev=c&hvqmt=e&keywords=structural+wood+filler&qid=1558799367&s=gateway&sr=8-1
If it looks rotten you may want to scrape out some of the crap and nuke it with a preservative before you cap or fill it.