I replaced the neem oil I used to use with sesame oil. It is just as effective as a preventative and it is something I already have for cooking purposes. It's also the active ingredient in Organocide 3 in 1 Bee Safe Garden Spray. I spray it every couple of weeks or so. I add a couple drops of liquid soap as a surfactant.
From the Organocide label:
Will NOT harm bees, butterflies or ladybugs.
Active Ingredients/Guaranteed Analysis:
0.08%..........Sesame Oil
Other Ingredients:
99.92%..........Lecithin, Edible Fish Oil, Potassium Sorbate, Water