sick swallowtail larvae

[Bughugger]

In rearing giant swallowtails, Papilio cresphantes, for the Houston Museum of
Natural Sciences Butterfly Center I am presented with a problem that has
everyone stumped.  In fact, we have lost a quarter of the larval population to
this conundrum.  The sympotoms are as follows:  upon reaching about the 3-4th
instar the larval integumen becomes slack and the catepillars fall to the soil
with osmetera extended.  Most lie on their backs with osmetera extended and
writhe around on the soil as if irritated by some unseen pest and eventually
dying.    I sent the larvae to a lab for analysis but the autopsies were
inconclusive.   They found no signs of bacterial, fungal or viral infection
and no signs of parasites or parasitoids.  They suggested that the problem was
environmental and suggested that I mist the larvae such that they get more
moisture; however, Houston while not be alot of things there is one thing that
it is and thats HUMID.

   If anyone has any thoughts or similar experiences please share them with
me.  

by-the way:
The larvae are feeding on Rue that was  grown organically from seed by our
horticulturists, we use no pesticides.  The last batch of Swallowtails that I
raised, fed on the same rue that Im using currently.   The Rue was washed with
alcohol about 3 months; however,  our 80 degree greenhouses assures that the
alcohol would surely have evaporated.  


Any and all  theories appreciated.

Sincerely,
Catherine Urban
Houston Museum of Natural Sciences, Cockrell Butterfly Center
713-649-4751