Hi Chao-Dong,

the best ways would be to extract DNA directly on field, or to keep samples frozen in nitrogen, but not easy to handle those equipments in travelling. is it possible for you to keep the insect alive, just the time to reach your lab?

fred

95% ethanol will just work fine for preserving DNA for genomic sequencing, say, on a illumina platform. To preserve high-quality DNA, make sure your samples do not exceed 1/3 of the volume of a vial. Say your vial is 50 ml, you will not want to have more than 15 ml of specimens in the vial. I also replace the ethanol with new one at the end of a day (if you have plenty of it). I personally find this the most important measure that I have taken to ensure high-quality DNA preservation, because it is not always easy to control how many specimens end up in a single vial. Keeping samples cool in the field is important too. It doesn't need to be 4 degree (like in a fridge). Room temperature will be just fine. Avoid exposure to sunlight. In terms of the number of specimens needed for DNA extraction, a single leg of a honeybee (10 mm) or a whole specimen of a small chalcid wasp (1-2 mm) is often enough. Liquid nitrogen will be difficult to handle and expensive to transport. Unless you are collecting specimens for a cryo-collection (deep freezing for many years), liquid nitrogen isn't really preferred. Just in case if you are interested in that options, there is a thing called dry shipper that will keep the specimens ultra-cold for up to 14 days, which is typically sufficient for transportation. 

Keep specimens individually in separate tubes to minimize the contamination, and changing ethanol as Guanyang suggested. Normally the concentration of ethanol is the higher the better, the temperature is the lower the better. But someone said multigelation is not good for the genome. So, if you don't have a serious freezer in the vehicle all the time, just keep the specimen cool or in room temperature instead of put them in different freezer every night and pull them out every day. 

If you have access to RNAlater, it will be best way to preserve insect sample. Ethanol works for some sample insects if you replace with fresh ethanol by the end of day. I have extracted DNA from inset as well as plant samples stored in ethanol. The yielding DNA were 200 to 300 bp in average size. That is not good for most PCR, if it is used for Illumina, the resulting assembly will be very low quality. RNAlater works for all insects. It provided good enough quality DNA for us to assemble 15 to 17 kb mitochondrial DNA sequences from Illumina runs.