I don't see why you could not do that.   I have used p. glycol pitfall traps over periods of several weeks in Nova Scotia and simply allowed small amounts of rain water to recharge the traps.  I don't think that P. glycol is likely to evaporate, at least not to any significant degree.  What evaporates is the water out of the solution.  Even if all the water in your traps evaporated you are still left with an effective killing solution.  I think the main reason we dilute P. glycol is because its expensive pretty much everywhere, and we stretch it usefulness.  In other words,  fill as many traps as we can, at minimal expense.  However, I've noticed the traps can start to smell pretty foul the more you reuse the same solution.

Best of luck,

Jason

Depending on how good the preservation effect of propylene glycol solution is (no idea, but ethylene glycol which I used in traps had little) and the interval you empty the trap, the solution might be very smelly after some time and this might increase with time. This will surely influence your trapping results since certain insect species are attracted by decay scent.

You can avoid this of course if you collect the insects with shorter intervals. And ethylene glycol might be a cheaper alternative (we used it because of low costs), if no vertebrates have access to the traps.

Hi Pete,

I've used 50% Propylene glycol in rounds of 21 days to look at grazing impacts on inverts, albeit in upland Britain and not in a tropical climate. I had small roof over each pitfall trap to stop rain water getting in but I think this also protected against evapouration. Evapouration was never a problem and specimens were always well preserved after 21 days. Soft bodied things did deteriorate and started to smell if traps became too dilute due to water getting in (if lids went missing) though.

But we also recycle used p. glycol to use for student pracs or dissertation projects. We sieve all of the used p.glycol and pool it and judge the quality of it by colour. When it is too dilute the colour changes from a nice clear blue (50%) to a dirty pale blue/green colour. If the colour is too poor we simply add more concentrate p.glycol to bring it back to an acceptable dilution (judged by colour). We find that we use much less 'new' p.glycol this way.

I have recycled pan trap and pitfall trap propylene glycol without any issues. In Costa Rica, the traps get smelly if left too long, but the strained fluid works fine and isn't smelly after a few rounds. We had small roofs over the traps in the tropics to prevent dilution with rain. We did not need this in summertime in Idaho. 

Thanks for your input everyone! 

thanks for sharing, I was looking for these information. But I would like to know, after removing the specimen from propylene glycol, can I keep it in ethnol while awaiting for identification? please any one to help