Phil,
  If the plastic bags are polyvinyl chloride based, these could be the
culprit.  On the other hand, cardboard boxes are notorious for being made
from inexpensive pulp and are often very acidic.  The typical corrugated
boxed should be avoided for long term storage, ie. boxes that most of us
would pick up in a grocery store, beer/soda pop flats, boxes that you
would pick up a U-Haul, etc.
  The way to mitigate damage, if you must use these types of containers,
is carefull control of the temperature and humidity where the collection
is stored.

Regards,
Charlie
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Charlie Sturm, Jr
Research Associate - Section of Invertebrate Zoology
                     Carnegie Museum of Natural History, Pittsburgh, PA
Assistant Professor - Family Medicine

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