Answer: Your concerns about using insecticides in ways not listed on the labels should be commended! Applying sprays when weather is hot can definitely lead to leaf burn, especially on tender-leaved plants such as roses. Bt is an excellent treatment for caterpillars and worms (it's a stomach poison), but it's not effective against adult beetles. Japanese beetle populations can be very destructive. Try applying milky spore disease to the soil to eliminate the grubs. You can also cultivate the soil frequently to expose the grubs to natural predators. The adults can be handpicked and destroyed, and you can purchase pheromone traps to trap the beetles.
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