I got an email from someone who reads this newsgroup who I know to be very
knowledgeable but who only lurks here. He suggested I consult my pest
management textbook on thrips with regard to their soil dwelling stage. The
text book points out that Thrips pupating in soil are very susceptible to
moisture and the associacated fungus problems. So in this respect, the
natural controls on them are just the opposite of fungus gnats. Damp
decomposing media posses a greater threat to their life-cycle than dry new
media. They can be easily drowned in just water and or fall prey to the
same decomposing fungal organisms that are breaking down the media. I
suspect thrips live cycle would be easily disrupted in the semi-hydro method
of potting orchids if they have no place else to pupate.

"Al" wrote in message
...
Once the third stage instar begins they stop eating some may even develop
wings, fly off or simply
crawl down into moderately or well decayed potting media to pupate. Like
fungus gnats, this instar requires potting media that is very moist and on
the decomposed side in order to do well. Dry and/or new potting media
will make it hard for both fungus gnats and 3rd stage thrip larvae to
survive in it.