It depends on how long the pitfall traps were open (in the field), i.e. how much of the alcohol had evaporated from your 50%-50% mixture.
To be on a safe side, I would add more alcohol to the mixture prior to sealing the cups, and then they can can keep for a long time. You should also check what is the optimal dilution ratio for you target organisms, most commonly this is around 70-80% (ethanol). Also watch out as having a wrong dilution can make your specimens brittle and cause them to break whille handling. Just to illustrate, I had kept ant samples in Brugal Rum for up to 5 months while doing fieldwork in the Dominican republic. Best Marko
