This is what I think I know (but I don't have Japanese Beetles or Iris Borers to speak of so I am no real authority on the subject) :
"Milky Spore" is a disease of the early stage larvae of Japanese Beetles only. It is also the name of the product that is used to cause the disease. The product is a bacteria--it is not an insecticide. The bacteria reproduce inside the larvae and when the larvae die more bacteria are released into the soil, so yes it is supposed to spread in the soil as years go by. However, the bacteria do not overwinter well in environments with cold winters so it is supposedly valuable to reapply if you have mating beetles after cold winters. It is good to apply late July/early August when the larvae are hatching and voraciously feeding close to the surface so that they can become infected.
The product, Milky Spore, has no effect whatsoever on Iris borers. I think there is a very different yet similar thing where nematodes, which are tiny worms, are added to soil to compromise the iris borers--and more frequent treatments are required. This could possibly be what Margie is thinking of...
Hope that helps.
Sorry to just barge in to ya'lls thread like that--
Beautiful roses everyone! I just found the July edition and I am duly impressed
