Tom Wahl wrote back and says this:
“I have read of neem oil being able to kill some insects, but I have never witnessed it myself. From what I have seen, it is an effective repellent against insect pests without harming non-pest insects. If I spray in on plums, grapes, and hazels just before the Japanese beetles arrive in mid/late June, they stay away. If I reapply it within two weeks, they keep on staying away. On the other hand, if I wait until the beetles show up and then apply neem, the beetles will fly away immediately (not die), but a few will start coming back after several days. Neem does not keep 100% of plum curculio away from plums and cherries (and it only takes one to ruin the fruit). I have not had much curculio problem on apples, probably because I have enough plums and cherries they feel no need to go to apples. Neem does seem to keep away apple maggots and coddling moth, which, for me, are more of a problem on apples. It is also effective at controlling cedar/apple rust and apple scab. The bottom line is that neem is great for most apple diseases and pests, but don’t count on it for curculio. FYI, there are a lot of products out there with neem oil cut with additives like surfactants and fillers. I don’t know how well they work. The neem I am talking about is 100% pure neem oil. One oz. oil plus one oz. dish soap to one gallon of water.”