Folklore says that after a heavy rainfall, you might as well get your sprayer out and reapply your insecticides. For those old twentieth century ­wettable powders, the folklore may be largely ­reliable, says Michigan State University entomologist Dr. John Wise. But it’s not necessarily true for ­twenty-first century materials. Many of the new reduced-risk insecticides have enhanced capacity to bind to plant surfaces or systemic activity within plant tissues, which can result in better rainfastness, and save a spray. For grape growers in the eastern half of North America, the vine-munching Japanese beetle provides virtually constant pressure from midsummer on. […]