I think Cynthia is absolutely right. I knew neem oil should not be exposed to light or heat. Because the active principle, azadirechtin, resembles a hormone. After reading Cynthia's thread I looked it up, biochemically speaking, and , of course the light/heat will render it ineffective by breaking up the highly oxidised parts ( hydroxyl and ketone ) of the molecule. I am sure light will disintegrate (reduction) the molecule, changing it so that the targeted organisms will not 'substitute' it in their metabolism.
I buy my Neem oil, dark and unsightly, in brown 75 ml bottles. If I open the cap one could smell the oil from about 3-4 metres.
linda shel: I use neem oil too. I even apply it right on the leaf with cotton swab. So far I have not had any problems except for the stench that lingers on a for a few hours..