A friend has a fig tree which seems to be the target of a worm. As far as I can see the leaves and fruit do not have any bite marks or worms/insects inside them, but the fruit born was less this year, and they mentioned that they think a worm or worms have burrowed into the tree's roots somehow and is killing it.
I researched and found that 'nematodes' are a common problem and sounded like the root of the problem, though they are apparently microscopic, and it seems like they have seen the worms themselves, so now I'm not so sure. As far as I know I don't think it is the borer/carpenter worms as there is no sawdust that they leave behind. The roots seem to be damaged almost as if medium sized parts of the bark is damaged.
Anyone have any ideas or know what can be done to cure the tree?