Yes, the borer has to come in "contact" with it.....that's why it's not as effective as the systemic poisons. Still, it does help...a little ! I use it on the iris that are grown next to "edibles', and on the re bloomers, when (if) they rebloom. I never see honey bees on spring bloom irises....I guess there are too many other things they would rather harvest....so I use the 'Merit" products in the spring, on them, but, late season blooms will attract the bees....so, I use the BT. I apply every week.....and try to get the solution to run down "into" the fan......probably not the best thing for rot prevention, but I haven't seen that soft rot is apparent in those sprayed that way
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You also have to spray "early", before they are into the rhizomes....so you need to start just as bloom is ready to end. That's the "good thing" about the systemics....no matter where he is, he'll get a dose.
