I’m answering on Kevin’s behalf, as he is no longer with the Museum. The neem oil was mixed with water and a little liquid castile soap added as an adjuvant. As I recall, the proportion used was an ounce to the gallon, but you will find doubtless find recommendations on any neem oil purchased. I have become wary of an indiscriminate use of neem oil, as it does have a residual effect and interferes with the program of beneficial insect release we now employ at The Cloisters. Our biggest pest problem in winter is with hard scale on woody plants, such as bay laurel and olive. We now release a tiny parasitic wasp with good effect. Once the wasps have thoroughly parasitized the scale, they do perish of starvation, and have to be re-introduced in case of re-infestation. Rather than applying neem oil to nearby rosemarys, I am experimenting with applying a pine sap-based anti-transpirant as a physical barrier against mildew and fungi. This is not a proven method—merely an experiment. Our rosemarys are not mildewed as yet, but it is early in the season. I find that they often hold their own until March, only to succumb.

If I were to use neem oil on a plant in the collection, I would isolate it from our other plants until I was reasonably sure that any residual effect that could effect beneficial insect populations had worn off.