I use neem oil for aphids and ants on my roses. I have a bottle that's about a year old. I just read that like other organic oils, it can go rancid. Does anyone know how long it should be stored? Or how to tell if rancid... it smells pretty bad normally?
In general anything that's a registered pesticide with the EPA has a shelf life of one year guaranteed, if it's less than that then I think they have to put an expiration date on it (any neem products you can buy should be EPA registered). Of course some things may last longer than that too, and I'm with you, I don't know how you would tell if it went rancid since it smells so bad in the first place!
Really hate it that I can't help you, for I've not found any specific mention of its 'shelf life' or how to tell when it may be rancid.
Altho' I did find some other info that mite help you in the future..
Quoting:TIPS ON USING NEEM OIL
1. Pure Neem Oil will retain its potency much longer if stored at about 40 F in a low light area such as a refrigerator.
2. Do not mix anything with Neem oil until you are ready to use it. Mix only the amount of Neem oil you will use in 4 to 6 hours.
3. A new batch of Neem oil, water and a little soap (according to the label) should be mixed each time you are going to spray.
4. The soap (dishwashing detergent) is used to help emulsify the oil. If no soap is used the Neem will not mix into solution with the water and spraying will not be effective.
5. Spray the complete plant including the top of the potting media. Spray benches, walkways and any surface over which an insect might travel.
6. A mixture of 1 oz. to 1 gallon of water should be used for spraying. A weaker solution may be used as a maintenance spray. It is impossible to give a definite schedule for spraying, however a "close eye" will help each person to adjust a timetable to maintain clean plants. You will probably not have to spray as often with Neem as with toxic insecticides.
* NOTE * The different Neem Oil products on the market may require handling different from what is described here. Please be sure to read the manufacturer's instructions carefully before use.
I'll keep my eyes peeled for any other info associated, tho' ..