borer id - help requested

[JC Cahill]

Hello,

I am a plant ecologist, rapidly becoming more interested in the ecology
of plant-insect interactions.  However, my botany training has some
limitations, and thus I am requesting help in getting started in the id
of an insect I collected.

In one study w/ the plant Apocynum cannibinum (in eastern PA), I found
that about 30% of all the plants (about 300) which I had marked had a
hole about 2-10 cm above the soil surface.  Generally this was
associated with a dead plant - standing dead despite having all its
leaves intact.  When I pulled up the plants, there were always hollow
down low, and once, I was able to collect a larvae of some stem borer.
Current belief is that this is a pyralid moth larvae.

A few questions:

1.  How would I go about attempting to fully id this critter if all I
have is a single larvae?  Collegues went to collect more this year, but
despite their frequency last year, they are basically absent from the
field this year.  (Big population crash?)  I imaging that since Apocynum
is in the milkweed family, and loaded w/ latex, not too many things can
feed in it.

2.  Although only about 30% of the plants had holes, about 75% of the
plants had "scars" near the base of the stem.  Basically what these
appeared to my eyes to be are areas in which some larvae was testing the
plant from the outside.  The scars were in a straight vertical line - as
though the larvae were crawling down and testing it as it went.  Does
this seem plausible?  What would the large holes be?  My guess is
emergence holes.  Are they likely to also be entry holes?

Any help would be much appreciated.

Thank You,

jc.cahill at ualberta.ca