I use 96% ethanol as a fixative, DNA can be isolated, even with long-term storage

95-97° Ethanol is always the best to preserve the specimens for a good DNA quality later on. As you mention it does evaporate quickly.

You can use ethelyn glycol, commonly used in pitfall traps and apparently it does not damage the DNA quality.

You can also try to use 80° ethanol and then change the material to 97°

here is more info that might help:

Article Comparison of preservation techniques for DNA extraction fro...

https://link.springer.com/article/10.1007%2Fs13127-010-0012-4

https://doi.org/10.1111/j.1570-7458.2010.00977.x

Hello Enrico; Santiago's suggestion would work. Your choice could include Ethylene Glycol, Propylene Glycol, or Ethyl Alcohol. Ant phylogenetic studies typically use specimens obtained using any of these. Ethylene glycol doesn't evaporate but is toxic if it is ingested by vertebrates. Propylene glycol is somewhat less volatile than alcohol and is not toxic.

I use ethylene glycol in pitfall traps and refresh those traps monthly in arid and semi-arid climates. Many of these specimens have been used in the kind of molecular studies you are considering. Best of luck, Jim Des Lauriers

All the ethanol solutions mentioned above are good for preserving DNA in samples. To avoid contamination, the attractant bait must be hung without touching the ethanol and it is necessary to place a plate over the pitfall trap so that water or dirt does not fall in the ethanol solution.

Thank you all for the detailed answers, I'll try your suggestions and let you know!