You should not spray insecticides while your trees are in bloom, you don’t want to kill your pollinators. From your description, it seems your pest is Oriental Fruit Moth. I spray my first insecticide spray when my nectarines are in shuck split, and this spray targets Plum Curculio and OFM. OFM has several generations, so I scout my orchard daily, and once I see a flagged shoot tip, I spray again and that takes care of the generation that’s just started attacking my trees. In east cost (where I am) university extensions recommend spraying every two weeks through spring and summer, which I do till early/mid July. Note that I bag my fruit, which protects them from late season insects (stink bugs, Japanese Beetles, hornets and later generations of OFM), so I don’t have to spray in the second half of the summer. Where you are in CA the pests and their pressure is different than here, so it is best to consult your local extension publications or experienced local fruit growers.
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