The effectiveness would be limited to the time the borer is "on the leaf". In order to "get the disease", the caterpillar would have to either ingest, or touch the "BT" ( I think). Also, the rain, or even a heavy dew, would tend to wash the BT from the leaf, although the residue left in the "leaf -fold"or whatever you call it, would be hard to wash off, and that's the area the borer travels on his way down the leaf. I don't spray it while ( or just before) bloom, because it's another thing that could be carried back to the hive, by the bees..and while it doesn't affect adult insects, anything still in the larval stage (worm) could be affected. I generally spray the iris at the same time I spray my "kale" crops...every 2-3 days, or so. One thing about the white, cabbage butterflies, that lay the eggs on the kale crops is, they are active in the daytime, so when you see them, you know you need to start sprying, and , by the number of them you see, you can gauge "how big' your problem is (or might be). The Moth that "is' the iris borer operates at night, or at least in low-light situations, and is sort of "camo" in color, so they can sneak up on you !! Still (IMHO) the best thing to do is inspect your plants for signs of damage, and keep the beds cleaned up! ............Arlyn
