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I am a gardener, not an entomologist not a forester.

It sounded like it (which is not a perjorative); I'm a sort of cross between the two.

"one would need an enormous count of these worms to do serious damage to an advanced infestation."

The point is that the beetles are propagating without nematodes while the nematodes can't propagate without beetles to eat. Mathematically that means they cannot eradicate but can only control a population.

I suppose--not know--that since the nematodes are parasitizing the grubs that their numbers will increase to match the available food source.

How effective this will be depends upon how far behind the curve one is when the nematodes are applied (breeding rate of nematodes v. beetles). It's a question of economics. I doubt that the nematodes are very good at moving large distances to find a new host without drying out or starving (which would of course, constitute nematode-cruelty; where's PETA when you need them? ;-), so, to be an effective control, one would need to cover the tree with only a few beetles in it to eat, which is an inefficient means of preventative control. If one waits until there are enough beetles to feed and propagate nematodes you are already behind the curve. IOW, it may work in your yard where you value highly each tree, but I question whether it is an economical control process on a landscape scale.

Good response, thank you. I value first-hand experience highly.

Thank you, happy to share it!

"The point is that the beetles are propagating without nematodes while the nematodes can't propagate without beetles to eat."

I would need more knowledge of entomology and the ecology of pine forests to respond. I know that the labs that produce them feed them on meal worms. They do not feed exclusively on pine borer grubs so this may not necessarily be true provided there is something else to eat somewhere on the tree (Pine sawyer larvae?).

"Mathematically that means they cannot eradicate but can only control a population."

In a true wilderness setting this might not be an issue. Some trees may die, and insect and woodpecker damage is tolerated. Hard to say how it would work for forestry purposes. It might be interesting to do a search and see if there have been any field trials of PN in large tracts of loggable timber.

(I wonder if BT would work on these grubs? Well, maybe for another thread...)