BSF: What barriers would they avoid?

So I am designing a bin and I want to use PVC as the tube to deliver the grubs to the chicken's .
I have read reports of before larvae not following the assigned path and climing any way they please.
I plan on discouraging this with a barrier. The only one. I have found so far that reportedly works is the 9 volt electric "fence" sometimes used against slugs.
Does anyone know of another barrier that works? I was considering petroleum jelly with hot pepper ...

I am fairly certain that I read a discussion of this issue on the Bio-Pod web site. I believe they found a strip of the hook side of hook and eye tape to be an effective barrier.

The issue is a mechanical one, as long as the larvae have a reasonably smooth surface to climb, they can slither right up a vertical face. Once they lose the adhesion they get on that smooth surface (I suspect it is a matter of surface tension) they will fall back.

Thank you Peter that was exactly what I was looking for!

The commercial biopod has a lip along the upper edge that would force a larva to crawl upside down to escape anywhere but along the designated ramp. Perhaps if the surface were completely wet, one could theoretically cling and move over this, but I doubt it. One would have to be able to crawl four or six inches along an upside-down surface to escape through the vent holes in the lid. I have hardly ever seen one even try to climb the near vertical walls of the pod below the lip, for that matter. But my pod tends to run dry.